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Title: Measures of irreversibility using quantum phase space
Date/Time: 28-Feb, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Irreversibility is an emergent concept, stemming from the complex interaction of a macroscopically large number of particles. It is also one of the most important concepts in thermodynamics, with ramifications in diverse fields. Understanding irreversibility from the microscopic level constitutes an active topic of research, particularly in the case of quantum system where irreversibility is known to emerge due to the entanglement between a system and its environment. In this lecture I will discuss recent results concerning the use of quantum phase space technique to establish measures of entropy production, the key quantifier of irreversibility. We will show how our formalism enables one to identify phase space currents operating both in the system and in the bath and which are ultimately responsible for the emergence of irreversibility. Our model also allows full control over the environment's degrees of freedom, which allows us to study also the interplay between irreversibility and the degree of non-Markovianity in a system. As we show, the system-environment mutual information is not a faithful witness of non-Markovianity, whereas the entropic distance of the environment from its initial state is. Finally, we will also discuss an application of these concepts to construct Onsager’s transport theory for the combined transfer of energy and squeezing in bosonic systems.
Title: Cyber-security in a quantum world
Date/Time: 20-Dec, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Quantum key distribution (QKD), promises in principle information-theoretic security based on the laws of physics, thus achieving the Holy Grail of communication security. Unfortunately, a practical QKD system rarely conforms to the idealized assumptions in security proofs. So, is QKD really secure in practice? Here, I survey the assumptions behind the security proofs of QKD and discuss about research efforts on quantum hacking and counter-measures about it. In particular, measurement-device-independent quantum key distribution (MDI-QKD) can automatically remove all side channel attacks on detectors, thus allowing the construction of QKD networks with *untrusted* relays. Recent results on MDI-QKD with asymmetric channel losses and twin-field (TF) type QKD protocols will also be discussed.
Title: Subwavelength optical barriers for cold atoms: creation and potential application
Date/Time: 22-Nov, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: The generation of subwavelength optical barriers on the scale of tens of nanometers, as conservative optical potentials for cold atoms, is discussed both theoretically and experimentally. In the proposed scheme they originate from nonadiabatic corrections to Born-Oppenheimer potentials for position-dependent “dark states” in atomic Λ configurations. The subwavelength optical barriers represent a “Kronig-Penney” potential, and I discuss the corresponding band structure including the effects of spontaneous emission and atom loss due to “bright” channels. Inclusion of an interparticle dipole-dipole interaction leads to formation of “domain wall molecules” and to unconventional Hubbard models with modulated in space interparticle interactions. As a brief discussion of potential applications, the subwavelength barrier can be used as a “splitter” to create a double-wire (or double-layer) with subwavelength spacing and, therefore, with substantially increased couplings as compared to ordinary optical lattices. As another application, specially designed subwavelength atomic internal-state spatial structures can be used for building an atomic scanning microscope with subwavelength resolution.
Title: Quantum Entanglement: Gaussian and Macroscopic
Date/Time: 18-Oct, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: In the first half of this talk I will discuss the quantification of entanglement in Gaussian systems and how this relates to channel simulation and error correction. We conclude that entanglement of formation is a more faithful measure of entanglement in Gaussian systems than negativity, both qualitatively and quantitatively, and illustrate this with examples. In the second half of this talk I will discuss the macroscopicity problem in quantum mechanics. If quantum theory is universal we would expect to be able to observe quantum effects such as superpositions on a macroscopic scale. The current inability to observe such effects is commonly attributed to decoherence, leading to a so-called "macro-scale" beyond which physical systems can be analysed without any reference to the quantum formalism. We challenge this view by showing that, with the assistance of a second system, a macroscopic system can be proved to be entangled even after arbitrary decoherence. We show this by introducing a modified Wigner's friend gedankenexperiment where the observer is not assumed to preserve quantum coherence.
Title: Magnetic coherence in spinor condensates and the ""hbar limit"" of field sensing
Date/Time: 13-Sep, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Precise measurement of magnetic fields is both a paradigmatic problem in quantum sensing, and a practical technology with many applications. In some parts of the magnetic sensing literature we find reference to an allegedly fundamental "energy resolution limit." This limit supposedly constrains the sensitivity of magnetometers to a value that contains only on the instrument’s time resolution (or bandwidth), its size, and fundamental constants. In particular the limiting noise spectral density, converted to a magnetostatic energy, is allegedly Planck's constant. Does such a fundamental limit really exist ? We will review the considerable but certainly not conclusive evidence for this "hbar limit," and show that such a limit is incompatible with our understanding of spinor Bose-Einstein condensates as described by well-known quantum gas and quantum sensing models. I will also describe our single-domain Rb-87 spinor condensate work, and prospects for an experiment to test the hbar limit.
Title: Quantum Simplicity: A tour of complexity science in a quantum World
Date/Time: 21-Jun, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Complexity and quantum science appear at first to be two fields that bear little relation. One deals with the science of the very large – seeking the understand how unexpected phenomena can emerge in vast systems consisting of many interacting components. Quantum theory, on the other hand, deals with particles at the microscopic level and is usually considered limited to the domain of individual photons and atoms. Yet, different as they appear, there is growing evidence that in interface ideas from quantum and complexity science, we may unveil new perspective in either both fields. In this presentation, I first introduce computational mechanics, a branch of complexity science captures structure by building the simplest causal models of natural observations. I then illustrate how many processes that require complex classical models may be simulated by remarkably simple quantum devices, and describe recent experiments to test this laboratory conditions. I then survey the potential consequences these developments to long-standing topics of interesting in both complexity and quantum science. The former in indicating that well-established notions of structure, complexity, may change when the quantum properties of information are taken into account. The latter in uncovering a new source of highlighting how a new source of temporal asymmetry can arise through classicalization. References: • Quantum mechanics can reduce the complexity of classical models, Nature Communications 3, 762 • Experimental quantum processing enhancement in modelling stochastic processes, Science Advances Vol. 3, no. 2, e160130. • Using quantum theory to simplify input–output processes. Nature partner journal: Quantum Information 3, 1 • A practical, unitary simulator for non-Markovian complex processes. arXiv:1709.02375 [To appear in Phys. Rev Lett] • Causal Asymmetry in a Quantum World. arXiv:1712.02368 [To appear in Physics. Rev. X]
Title: Superconducting qubits, the macroscopic atoms for building quantum processors
Date/Time: 28-Mar, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Quantum theory, formed in the early part of the last century, has revolutionized our view on the nature of physical reality. More than half a century after its inception, a few great minds of physics, including Richard Feynman, predicted that the laws of quantum mechanics could give rise to a computing paradigm that is far superior to classical computing for certain tasks. Decades have passed since their great insight, but controlling fragile quantum systems well enough to implement even the most primitive quantum computer has proven difficult. A promising way of making quantum bits (qubits) is by using superconducting Josephson junctions. Devices made out of these junctions show quantum properties at the macroscopic level, providing advantages in controlling and connecting qubits. In this talk, I discuss the prospects and challenges in making quantum processors by using superconducting qubits. I report on our progress at Google and explain how qubits can be used to study problems at the core of statistical mechanics; in particular, we studied the signatures of transition from ergodic to the many-body localized state in a chain of 9 qubits.
Title: Cold dipolar bosons
Date/Time: 01-Mar, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: I will review recent research on the role of long range forces in quantum gases. I will mention experimental efforts in condensing chromium, erbium and dysprosium. I will describe in some detail the properties of just two dipolar atoms in a trap comparing magnetic and electric dipolar systems. Then I will talk about dipolar dark solitons, ending with our construction of a multi-particle roton state.
Title: Modes and States in Quantum Optics
Date/Time: 31-Jan, 03:30PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Quantum Optics, as the child of Optics and Quantum Mechanics, has inherited a double linearity: that of Maxwell equations, which use optical modes as a basis of solutions, and that of the Schrödinger equation, which uses quantum state bases. Considering these two bases on an equal footing and tailoring quantum fields not only in given modes, but also optimizing the spatiotemporal shapes of the modes in which the state is defined, opens new ways for characterizing and exploiting complex quantum states. We will describe several applications of this approach in quantum information processing and quantum metrology.
Title: Scalable quantum computing withsimple and complex atoms
Date/Time: 18-Jan, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Quantumcomputing is a few decades old and is currently an area where there is greatexcitement, and rapid developments. A handful of distinct approaches have shownthe capability of on demand generation of entanglement and execution of basicquantum algorithms.
Oneof the daunting challenges in developing a fault tolerant quantum computer isthe need for a very large number of qubits. Neutral atoms are one of the mostpromising approaches for meeting this challenge. I will give a snapshot of thecurrent status of atomic quantum computing, describe the physics underlyingneutral atom qubits and Rydberg state mediated quantum gates, and show how oneof the most complicated atoms in the periodic table may lead to some simplesolutions to hard problems.
Title: Foundations of Lattice-based Cryptography
Date/Time: 23-Nov, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Lattice-based cryptosystems are perhaps the most promising candidates for post-quantum cryptography as they have strong security proofs based on worst-case hardness of computational lattice problems and are efficient to implement due to their parallelizable structure. Attempts to solve lattice problems by quantum algorithms have been made since Shor’s discovery of the quantum factoring algorithm in the mid-1990s, but have so far met with little success if any at all. The main difficulty is that the periodicity finding technique, which is used in Shor’s factoring algorithm and related quantum algorithms, does not seem to be applicable to lattice problems. In this talk, I will survey some of the main developments in lattice cryptography over the last decade or so. The main focus will be on the Learning With Errors (LWE) and the Short Integer Solution (SIS) problems, their ring-based variants, their provable hardness under the intractability assumptions of lattice problems and their cryptographic applications.
Title: CQT Colloquium by Nobuyuki Imoto, Osaka University
Date/Time: 02-Nov, 04:00PM
Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: If a high-quality entanglement is shared between two distant parties, there are a lot of innovative things we can do such as device-independent QKD, which is in fact initiated by Ekert [1]. For this, faithful quantum communication via a noisy and lossy channel is an important element, which is the main research item of my research group [2]. As we proceed this type of research, we encounter some fundamental research themes. For example, we developed a frequency converter of a single photon that maintains coherence and entanglement [3]. Our frequency converter acts as a beamsplitter in frequency domain, whose conversion efficiency is tunable. Using this frequency-domain beamsplitter, we performed Hong-Ou-Mandel interference [4], where two different-color input photons are converted into two same-color output photons whose color stochastically becomes either of the original two colors. We also performed Mach-Zehnder interference [5], which, in frequency domain, is more difficult than the HOM interference. The second example of fundamental research themes, other than the frequency converter, is cheat-sensitive type communication [6], where we can guess the result of the measurement performed by our partner regardless whether he/she chose from the linear and circular polarization measurements, which at first glance appears to be in conflict with the uncertainty principle. The key is that we not only prepare the initial state of the photon before the partner’s measurement but also measure its final state after the partner’s measurement. This concept is generalized to so-called weak value [7], and we are pursuing the meaning and usage of this new concept [8]. [1] A. K. Ekert, PRL67, 661 (1991). [2] T. Yamamoto et al., Nature 421, 343-346 (2003); Nat. Photon. 2, 488 - 491 (2008). [3] R. Ikuta et al., Nat. Commun. 2, 1544 (2011). [4] T. Kobayashi et al., Nat. Photon. 10, 441–444 (2016). [5] T. Kobayashi et al., Optics Express, 25, 012052_1_9 (2017). [6] K. Shimizu et al., PRA84, 022308 (2011). [7] Y. Aharonov et al., PRL, 60, 1351 (1988). [8] K. Yokota et al., New J. Phys. 18, 123002 (2016).
Title: Quantum optics with Rydberg atoms
Date/Time: 26-Oct, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: There have been growing research activities involving Rydberg atoms in different directions of quantum optics, both fundamental and applied. In this talk, I first briefly review a few systems and examples, which exploit the exotic properties of Rydberg atoms for new phenomena and applications. I then discuss some of our experimental efforts, including electromagnetically induced transparency and microwave-optical conversion using Rydberg atoms.
Title: Thermodynamics of Quantum Devices
Date/Time: 19-Oct, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Quantum thermodynamics addresses the emergence of thermodynamical laws from quantum mechanics. The viewpoint advocated is based on the intimate connection of quantum thermodynamics with the theory of open quantum systems. Quantum mechanics inserts dynamics into thermodynamics giving a sound foundation to finite-time-thermodynamics. The emergence of the 0-law I-law II-law and III-law of thermodynamics from quantum considerations will be presented through examples. I will show that the 3-level laser is equivalent to Carnot engine. I will reverse the engine and obtain a quantum refrigerator. Different models of quantum refrigerators and their optimization will be discussed. A heat-driven refrigerator (absorption refrigerator) is compared to a power-driven refrigerator related to laser cooling. This will lead to a dynamical version of the III-law of thermodynamics limiting the rate of cooling when the absolute zero is approached. The thermodynamically equivalence of quantum engines in the quantum limit of small action will be discussed. I will address the question why we find heat exchangers and flywheels in quantum engines. I will present a molecular model of a heat rectifier and a heat pump in a non-Markovian and strong coupling regime.
Title: Evolution and Perspective of Planar Waveguide Devices
Date/Time: 07-Sep, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The talk will review progress and future prospects of planar waveguide devices. Silica-based PLCs (planar lightwave circuits) and InP PICs (photonic integrated circuits) are widely used in the current WDM and FTTH systems. The success of silica PLCs and InP PICs strongly depends on their well controlled core geometries and refractive-index uniformities. On the other hand silicon photonics is widely regarded as a promising technology to meet the requirements of rapid bandwidth growth and energy-efficient communications while reducing cost per bit. One of the most prominent advantages of photonics interconnection over metallic interconnects is higher bandwidth and signal routing functionality using WDM technology. Expectations on Si photonics and technical challenges for silicon photonics will be described.
Title: What do the data tell us?
Date/Time: 31-Aug, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: We gather information about physical systems by observation. In the realm of quantum physics, the experiments give us probabilistic data with natural statistical fluctuations that cannot be reduced by better instrumentation. What do such data tell us about the quantum system under study? A systematic and reliable answer can be given with the methods of quantum state estimation and quantum parameter estimation. I will report on recent developments.
Title: Quantum simulations with strongly interacting photons: Merging condensed matter with quantum optics for quantum technologies
Date/Time: 27-Jul, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Classical computers require enormous computing power and memory to simulate even the most modest quantum systems. That makes it difficult to model, for example, why certain materials are insulators and others are conductors or even superconductors. R. Feynman had grasped this since the 1980s and suggested to use instead another more controllable and perhaps artificial quantum system as a "quantum computer" or specifically in this case a "quantum simulator". Working examples of quantum simulators today include extremely cold atoms trapped with lasers and magnetic fields and ions in electromagnetic traps. Photons and polaritons in light-matter systems have also recently emerged as a promising avenue especially for simulating out of equilibrium many-body phenomena in a natural driven-dissipative setting. I will briefly review in non-specialist terms the main results in this area including the early ideas on realizing Mott insulators, Fractional Hall states and Luttinger liquids with photons [1,2,3]. After that I will present in more detail a recent experiment in many-body localization physics using interacting photons in the latest superconducting quantum chip of Google [4]. A simple method to study the energy-levels-and their statistics - of many-body quantum systems as they go through the ergodic to many-body localized (MBL) transition, was proposed and implemented. The formation of a mobility edge of an energy band was observed and its shrinkage with disorder toward the center of the bands was measured, a direct observation of a canonical condensed matter concept perhaps for the first time. Beyond the applications in understanding fundamental physics, the potential impact of this field in different areas of quantum and nano technology and material science will be touched upon. References 1. D.G. Angelakis and C. Noh “Many-body physics and quantum simulations with light” Report of Progress in Physics, 80 016401 (2016) 2. "Quantum Simulations with Photons and Polaritons: Merging Quantum Optics with Condensed Matter Physics" edited by D.G. Angelakis, Quantum Science and Technology Series, Springer International Publishing, 2017, ISBN 978-3-319-52023-0, DOI 10.1007/978-3-319-52025-4 3. Keil, Noh, Rai, Stutzer, Nolte, Angelakis, A. Szameit "Optical simulation of charge conservation violation and Majorana dynamics", Optica 2, 454 (2015) 4. P. Roushan, C. Neill, J. Tangpanitanon,V.M. Bastidas,, …, D.G. Angelakis, J. Martinis. “Spectral signatures of many-body localization of interacting photons”, under review
Title: Secure quantum computation
Date/Time: 18-May, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The realisation that conventional information theory and models of computation do not account for the full generality of states and operations described by quantum mechanics has led to the burgeoning field of quantum information processing. By harnessing quantum phenomena it is possible to produce stronger forms of cryptography and more efficient algorithms than could exist in a purely classical world. Computer security lies at the intersection of computation and cryptography, and has become an increasingly important topic in recent years. Since quantum information processing leads to advantages in cryptography and computation separately, it is natural to ask whether it may also enhance computer security. In this talk I will argue that the answer to this question is a resounding “yes”, and discuss recent developments in the field.
Title: The applied side of Bell nonlocality
Date/Time: 27-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Since its formulation in 1964, Bell's theorem has been classified under "foundations of physics". Ekert's 1991 attempt to relate it to an applied task, quantum cryptography, was quenched by an approach that relied on a different basis and was allegedly equivalent. Ekert's intuition was finally vindicated with the discovery of "device-independent certification" of quantum devices. In this colloquium, I shall revisit the tortuous history of that discovery and mention some of the subsequent results. Some references that review this topic: V. Scarani, Acta Physica Slovaca 62, 347 (2012) [https://arxiv.org/abs/1303.3081] N. Brunner et al., Rev. Mod. Phys. 86, 419 (2014) [https://arxiv.org/abs/1303.2849] S. Pironio et al., New J. Phys. 18, 100202 (2016) [http://iopscience.iop.org/1367-2630/focus/Focus-on-Device-Independent-Quantum-Information]
Title: Quantum Physics: A Possible Theory of the World as a Whole
Date/Time: 23-Mar, 04:00AM
Venue: CQT Seminar Room, S15-03-15
Abstract: Quantum mechanics is commonly said to be a theory of microscopic things: molecules, atoms, subatomic particles. Most physicists, though, think it applies to everything, no matter what the size. The reason its distinctive features tend to be hidden is not a simple matter of scale. Over the past few years experimentalists have seen quantum effects in a growing number of macroscopic systems. The quintessential quantum effect, entanglement, can even occur in large systems as well as warm ones - including living organisms - even though molecular jiggling might be expected to disrupt entanglement. I will discuss how techniques from information theory, quantum and statistical physics, can all be combined to elucidate the physics of macroscopic objects. Can it be that part of the macroscopic world is quantum, while the rest is, in some sense, classical? This question is also of fundamental importance to the development of future quantum technologies, whose behavior takes place invariably in the macroscopic non-equilibrium quantum regime. I will discuss the concept of quantum macroscopicity and argue that it should be quantified in terms of coherence based on a set of conditions that should be satisfied by any measure of macroscopic coherence. I will show that this enables a rigorous justification of a previously proposed measure of macroscopicity based on the quantum Fisher information. This might shed new light on the standard Schrödinger cat type interference experiment that is meant to demonstrate the existence of macroscopic superpositions and entanglement.
Title: Holographic quantum error-correcting codes
Date/Time: 02-Feb, 12:00AM
Venue: CQT Seminar Room, S15-03-15
Abstract: In this talk, I will explore the recent connection between two profound ideas, quantum error correction and holography. The first, represents the realization that reliable quantum information processing could be achieved from imperfect physical components. The second, is a duality between two physical systems on different spatial dimensions which may be identified leading to the exact same predictions. Notably, only one of the two systems explicit includes gravitational features. Recently, quantum information has emerged as a natural tool to relate these two descriptions. As such, concepts familiar to quantum information scientists such as entanglement, compression and quantum error correction are playing important roles in understanding this duality. Conversely, the holographic duality is proposing a new lens through which to explore aspects of quantum error correction. In this talk, I will introduce some of the properties imposed by holography on corresponding quantum error-correcting codes, describe explicit tensor network codes which exhibit some of these properties and explore the implications of holographic predictions from a code-theoretic perspective.
Title: Demon Dynamics: Deterministic Chaos, the Szilard Map, and the Intelligence of Thermodynamic Systems
Date/Time: 13-Jan, 12:00AM
Venue: CQT Seminar Room, S15-03-15
Abstract: We introduce a deterministic chaotic system—the Szilard Map—that encapsulates the measurement, control, and erasure protocol by which Maxwellian Demons extract work from a heat reservoir. Implementing the Demon's control function in a dynamical embodiment, our construction symmetrizes Demon and thermodynamic system, allowing one to explore their functionality and recover the fundamental trade-off between the thermodynamic costs of dissipation due to measurement and due to erasure. The map's degree of chaos—captured by the Kolmogorov-Sinai entropy—is the rate of energy extraction from the heat bath. Moreover, an engine's statistical complexity quantifies the minimum necessary system memory for it to function. In this way, dynamical instability in the control protocol plays an essential and constructive role in intelligent thermodynamic systems.
Title: From quantum philosophy to quantum technology
Date/Time: 07-Dec, 06:00PM
Venue: Auditorium 1 Level 1, Town Plaza University Town, NUS
Abstract: Quantum physics has become one of the best confirmed scientific theories ever devised by mankind and it is firmly embedded in many modern technologies. The fact that quantum concepts often seem to contradict traditional notions of reality, space, time or logic has inspired fundamental philosophical debates as well as even further intriguing developments in quantum computing, communication, simulation, sensing and metrology. Based on a tutorial review of the state of the art in the field, we will focus on the concept of matter-waves, which was originally put forward by Louis de Broglie in 1923 “to solve almost all the problems brought up by quanta”, and cast in mathematical form by Erwin Schrödinger in 1926. We will see how modern matter-wave interferometry can serve in sophisticated tests of fundamental physics and as a subtle force sensor with many interdisciplinary applications.
Title: The Quantum Approximate Optimization Algorithm: A Good Choice to Run on a Near Term Quantum Computer
Date/Time: 07-Dec, 04:30PM
Venue: Auditorium 1 Level 1, Town Plaza University Town, NUS
Abstract: I will describe a quantum algorithm for approximate optimization and explain how to analyze its performance on all instances of particular combinatorial optimization problems. I will explain why this algorithm is well suited to run on small scale quantum computers because of its low circuit depth and simple gate structure. I will also explain how, in principle, running this algorithm can demonstrate Quantum Supremacy because if a classical algorithm could efficiently sample its output then the Polynomial Hierarchy would collapse.
Title: Exploring quantum matter with photons
Date/Time: 17-Nov, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: From the beginning of the 20th century, photons have provided a paradigm for the crucial effects of quantum mechanics. In this century, advances in quantum devices have led the way to strong photon-photon interactions, merging the best coherence properties of light with many-body physics. I will discuss our efforts for realizing nontrivial phases of matter using optical- and microwave-domain photons. While our ability to control the single-particle properties of light via photonic crystal and metamaterial techniques provides a useful tool kit for single-particle physics, going to the many-body regime has crucial challenges to be overcome. I will emphasize how we can overcome these difficulties, by developing strong nonlinearities with light, creating a chemical potential for photons, and extending these ideas into other gauge theories.
Title: Magnetic imaging with point defects in diamond
Date/Time: 10-Nov, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The ability to quantitatively map magnetic field distributions is of crucial importance for fundamental studies ranging from materials science to biology, and for the development of new devices e.g. in spintronics. Recently it has been demonstrated that scanning magnetometry based on a single spin associated to an impurities hosted in a solid is an efficient technique which combines high sensitivity and nanoscale resolution. The sensing signal relies on the optical detection of the electron spin resonance associated with a single nitrogen-vacancy (NV) center in diamond attached to a AFM tip. The magnitude of the stray magnetic field above a magnetic sample can then be determined from the Zeeman shifts of the energy levels associated to this artificial atom in the solid state. Extending this technique to a cryogenic environment will open the way to investigate many magnetic phenomena occuring in complex condensed matter systems, such as superconductivity or the magnetic properties of strongly correlated systems. I will present our recent realizations of a scanning magnetometer based on NV centers in a nanodiamond grafted at the apex of a AFM tip, and how they have been applied to the imaging of magnetic nanostructures. I will also describe how NV centers can be efficiently engineered by combining plasma-assisted diamond growth and nanoscale ion implantation.
Title: Antihydrogen - a tool to study matter-antimatter symmetry in the laboratory
Date/Time: 22-Sep, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Antihydrogen, the bound state of an antiproton and a positron, is the simplest atom consisting purely of antimatter. Its matter counterpart, hydrogen, is one of the best studied atomic systems in physics. Thus comparing the spectra of hydrogen and antihydrogen offers some of the most sensitive tests of matter-antimatter symmetry. Furthermore, the availability of neutral antimatter offers for the first time a precise measurement of its gravitational interaction that was so far not possible due to the dominance of the electro-magnetic interaction for charged antiparticles. The formation and experimental investigation of antihydrogen is the main physics goal of several collaborations at the Antiproton Decelerator of CERN. The ASACUSA collaboration is pursuing a measurement of the ground-state hyperfine structure of antihydrogen in an atomic beam, a quantity which was measured in hydrogen using a maser to a relative precision of 10^{-12}. The AEgIS collaboration aims at using an ultra-cold beam of antihydrogen atoms and a classical moiré deflectometer to determine the gravitational interaction between matter and antimatter in a first step to several percent precision. After a first production of cold antihydrogen in 2002 and a first trapping in 2010 the experiments are still in the process of optimising the antihydrogen production from trapped antiprotons and positrons. The status and prospect of these experiments will be reviewed.
Title: Quantum coherence in photosynthetic proteins: insights for emerging energy technologies
Date/Time: 28-Jul, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Coherence beating has been observed in two dimensional optical spectroscopy of several photosynthetic proteins ranging from light harvesting antennae isolated from algae, plant and bacteria to photosynthetic reaction centres isolated from plants. For the majority of these complexes the leading hypothesis for the physical mechanism supporting coherence beating is an intertwined electronic and vibrational dynamics whereby a single quanta of energy is quasi-coherently shared between these degrees of freedom. Does this mechanism lead to a similar or different picture for the energetics in these biological complexes? In this lecture I will discuss our research efforts towards addressing this question and the lessons we may learn for emerging bio-inspired energy technologies.
Title: Fundamental tests of nature with cooled and stored exotic ions
Date/Time: 14-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The presentation will concentrate on recent applications with exciting results of Penning traps in atomic and nuclear physics with cooled and stored exotic ions. These are high-accuracy mass measurements of short-lived radionuclides, g-factor determinations of the bound-electron in highly-charged, hydrogen-like ions and g-factor measurements of the proton and antiproton. The experiments are dedicated to nuclear-, neutrino- and astrophysics studies in the case of mass measurements on radionuclides, and to the determination of fundamental constants and a CPT test using g-factor measurements.
Title: Shedding Starlight on the Quantum Fabric of Spacetime
Date/Time: 07-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Over the last decade there has been an intense effort using observations in astrophysics for setting constraints, with Planck-length sensitivity, on the properties of the laws of propagation of particles in a quantum spacetime. Importantly this has led to the demise of an old-fashioned naïve assessment according to which Planck-scale effects could never be tested and research in quantum gravity should be confined to the realm of pure mathematics. I stress that the next few years might provide another formidable boost for this research area. For studies of systematic effects on particle propagation an exciting new window will result from "multimessenger analyses", combining information obtained with gamma rays, cosmological neutrinos and gravity waves. For studies of the effects of "fuzziness" (non-systematic quantum-spacetime effects) significant improvements are expected for searches of the associated decoherence effects.
Title: Photonic Crystals and Photonic Molecules at Telcom Wavelengths
Date/Time: 30-Mar, 12:00PM
Venue: Photonic Crystals and Photonic Molecules at Telcom Wavelengths
Abstract: I will discuss the use of defects in photonic crystal waveguides to creates optical cavities which can control the emission of single quantum dots at telecom wavelengths. These waveguide structures enable complex geometries to be used in coupling two or more cavities together to produce photonic molecules. I will focus the talk on investigations of the mode splitting in a photonic molecule consisting of two coupled photonic crystal cavities separated by an optical well in a photonic crystal waveguide. Using a confocal microphotoluminescence mapping technique I will show that fine control of the coupling between the cavities can be achieved by the addition of an optical well. It is notable that an increase in the depth of the well results in an increased mode splitting and a strongly red shifted symmetric supermode (ground mode). Photonic molecules have recently been proposed for applications in cavity quantum electrodynamics (cQED) including the production of strong photon antibunching. These however require great control of the coupling strength between coupled cavities. We can finely tune the coupling between cavities embedded in a photonic crystal waveguide. Each cavity is formed by the local modulation of the waveguide width, which effectively defines two optical wells. A third narrower optical well is created by locally increasing the waveguide width between the cavities as shown by the red circles in figure (a). The coupled system studied here supports four supermodes as shown in figures (b)-(c).
Title: Leibniz on Complexity
Date/Time: 10-Mar, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: 2016 is the tercentenary of the death of the remarkable philosopher/mathematician Leibniz. In this talk we shall present an appreciation of his work on information, computation and complexity leading up to modern work on algorithmic information and conceptual complexity, with applications in epistemological critiques of physics, mathematics and biology.
Title: Random words, longest increasing subsequences, and quantum PCA
Date/Time: 18-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Suppose you have access to i.i.d. samples from an unknown probability distribution $p = (p_1, …, p_d)$ on $[d]$, and you want to learn or test something about it. For example, if you wants to estimate $p$ itself, then the empirical distribution will suffice when the number of samples, $n$, is $O(d/epsilon^2)$. In general, you can ask many more specific questions about $p$: Is it close to some known distribution $q$? Does it have high entropy? Etc. For many of these questions the optimal sample complexity has only been determined over the last $10$ years in the computer science literature. The natural quantum version of these problems involves being given samples of an unknown $d$-dimensional quantum mixed state $\rho$, which is a $d \times d$ PSD matrix with trace $1$. Many questions from learning and testing probability carry over naturally to this setting. In this talk, we will focus on the most basic of these questions: how many samples of $\rho$ are necessary to produce a good approximation of it? Our main result is an algorithm for learning $\rho$ with optimal sample complexity. Furthermore, in the case when $\rho$ is almost low-rank, we show how to perform PCA on it with optimal sample complexity. Surprisingly, we are able to reduce the analysis of our algorithm to questions dealing with the combinatorics of longest increasing subsequences (LISes) in random words. In particular, the main technical question we have to solve is this: given a random word $w \in [d]^n$, where each letter $w_i$ is drawn i.i.d. from some distribution $p$, what do we expect $\mathrm{LIS}(w)$ to be? Answering this question requires diversions into the RSK algorithm, representation theory of the symmetric group, the theory of symmetric polynomials, and many other interesting areas.
Title: Computing beyond the Age of Moore’s Law
Date/Time: 07-Dec, 06:00PM
Venue: University Hall Auditorium Level 2, Lee Kong Chian Wing University Hall, NUS
Abstract: With the end of Moore’s Law within sight (really!), what opportunities exist to continue the exponential scaling of computer performance and efficiency into the future? The primary consumers of energy and time in computers today are not the processors, but rather the communication and storage systems in a data center. Even if it were possible to perform computation infinitely fast with zero energy consumption, there would be little change in the power and time required to perform a calculation on the types of ‘big data’ sets that are beginning to dominate information technology. I will describe a path forward with two stages. The first is to change the fundamental structure of a computer from the processor-centric von Neumann paradigm that has dominated for the past seven decades to a memory-driven architecture based on a flat nonvolatile memory space, high bandwidth photonic interconnect and dispersed system-on-chip processors. This is the vision behind The Machine, a major research and development effort currently in progress in Hewlett Packard Enterprise. Once this transformation has been completed, a new era of hybrid computation can begin, which is the motivation behind a new Nanotechnology-Inspired Grand Challenge for Future Computing recently announced by the US White House. One of the drivers for this initiative is the thesis that although our present understanding of the brain is limited, we know enough now to design and build circuits that can accelerate certain computational tasks; and as we learn more about how brains communicate and process information, we will be able to harness that understanding to create a new exponential growth path for computing technology.
Title: Certified Quantum Randomness
Date/Time: 07-Dec, 04:30PM
Venue: University Hall Auditorium Level 2, Lee Kong Chian Wing University Hall, NUS
Abstract: Randomness is a physical phenomena which we are confronted with all the time. Will it rain today? At what time? Will the train be on time? But are such phenomena truly random? Good randomness is essential for many applications. Cryptography, the art of hiding information from malicious users, is only as good as the source of randomness that underlies it. Quantum mechanics, the theory of microscopic phenomena, can only predict the probability of events: for instance quantum theory can only predict the probability that a radioactive nucleus will decay, not if the nucleus will decay. Does this mean that microscopic phenomena are truly random? By studying systems of two entangled particles, it can be shown both theoretically and experimentally, that events at the microscopic scale are truly random, truly unpredictable. Beyond its philosophical implications, these works also have important potential applications. Indeed they imply that one can build random number generators that certify that they work correctly. That is, if the random number generator malfunctions in some way, if the numbers it produces cease to be random, this will automatically be detected. By extending this idea, one could also build quantum cryptographic systems and quantum computers that certify that they work correctly. We discuss the perspectives for practical implementations.
Title: Black Holes, Firewalls, and the Complexity of States and Unitary Transformations
Date/Time: 20-Aug, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I'll discuss some recent results, motivated by the black-hole firewall paradox and the AdS/CFT correspondence, about the quantum circuit complexity of preparing certain entangled states and implementing certain unitary transformations. One result is a strengthening of an argument by Harlow and Hayden: I'll show that, under plausible assumptions, "decoding" useful information from Hawking radiation, as called for by the AMPS "firewall" thought experiment, requires the computational power to invert arbitrary cryptographic one-way functions, something we think not even quantum computers could do in sub-astronomical time. A second result, joint with Lenny Susskind, considers the circuit complexity of the kinds of states that could arise in AdS/CFT, and shows that, under a reasonable conjecture about complexity classes (PSPACE is not in PP/poly), the complexity indeed becomes superpolynomially large, as predicted by a conjectured relationship between complexity and geometry. I'll also discuss more general problems about the complexities of states and unitary transformations, which I find fascinating even apart from the quantum-gravity motivation.
Title: A single charge in a Bose-Einstein condensate: from two to few to many-body physics
Date/Time: 28-May, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Electrons attract polarizable atoms via a 1/r^4 potential. For slow electrons the scattering from that potential is purely s-wave and can be described by a Fermi pseudopotential. To study this interaction Rydberg electrons are well suited as they are slow and trapped by the charged nucleus. In the environment of a high pressure discharge Amaldi and Segre, already in 1934 observed a lineshift proportional to the scattering length [1]. At ultracold temperatures and Rydberg states with medium size principle quantum numbers n, one or two ground state atoms can be trapped in the meanfield potential created by the Rydberg electron, leading to so called ultra-long range Rydberg molecules [2]. At higher Rydberg states the spatial extent of the Rydberg electron orbit is increasing. For principal quantum numbers n in the range of 100-200 and typical BEC densities, up to several ten thousand ground state atoms are located inside one Rydberg atom, We excite a single Rydberg electron in the BEC, the orbital size of which becomes comparable to the size of the BEC. We study the coupling between the electron and phonons in the BEC [3]. We also observe evidence for ultracold collisions involving a single ion which is shielded by a Rydberg electron. Reactive processes due to few-body Langevin dynamics are mostly l-changing and lead to molecule formation. As an outlook, the trapping of a full condensate inside a Rydberg atom of high principal quantum number, the imaging of the Rydberg electron's wave function by its impact onto the surrounding ultracold cloud as well as the observation of polaron formation seem to be within reach [4]. [1] E. Amaldi and E. Segre, Nature 133, 141 (1934) [2] C. H. Greene, et al., PRL 85, 2458 (2000); V. Bendkowsky et al., Nature 458, 1005 (2009) [3] J . B. Balewski, et al., Nature 502, 664 (2013) [4] T. Karpiuk, et al., arXiv:1402.6875
Title: Dipole QED: an alternative paradigm for quantum non-linear optics and non-equilbrium dynamics
Date/Time: 16-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: In cavity QED (cQED), mirrors alter the electromagnetic field in the vicinity of an emitter and thereby modify the light-matter interaction in a significant way. In the strong coupling regime, the effect of the cavity dominates over the coupling to the vacuum. Similarly in dipole QED (dQED), one or more dipoles in the vicinity of the emitter modifies both the vacuum coupling (sub- or super-radiance) and the resonant frequency. The condition for strong coupling is simply that the dipoles must be closer than the reduced wavelength of emission, which is of order tens of nanometers for optical emitters [1] or millimeters for microwave emitters, e.g. transitions between highly-excited Rydberg states in atoms [2]. We will discuss experiments in both these regimes [1,2] and then discuss applications of dQED in single photon non-linear optics [3,4], and non-equilbrium systems with long-range interactions [5].
Title: Random for whom?
Date/Time: 08-Dec, 06:00PM
Venue: Ngee Ann Kongsi Auditorium Level 2 Education Resource Centre University Town, NUS
Abstract: There is a difference between the impossibility of predicting the weather and the impossibility of giving a value to both position and momentum of an electron. This difference is what makes quantum physics hard to understand: intrinsic randomness. In this talk, I shall try to clarify what "intrinsic randomness" means, how we can be sure that it is "real" and not an artefact of an imperfect theory. Besides shaping our view of the natural world in an unexpected way, the existence of intrinsic randomness may even lead to practical benefits
Title: The Quantum Way of Doing Computations
Date/Time: 08-Dec, 05:00PM
Venue: Ngee Ann Kongsi Auditorium Level 2 Education Resource Centre University Town, NUS
Abstract: Since the mid nineties of the 20th century it became apparent that one of the centuries’ most important technological inventions, computers in general and many of their applications could possibly be further enormously enhanced by using operations based on quantum physics. This is timely since the classical roadmaps for the development of computational devices, commonly known as Moore’s law, will cease to be applicable within the next decade due to the ever smaller sizes of the electronic components that soon will enter the quantum physics realm. Computations, whether they happen in our heads or with any computational device, always rely on real physical processes, which are data input, data representation in a memory, data manipulation using algorithms and finally, the data output. Building a quantum computer then requires the implementation of quantum bits (qubits) as storage sites for quantum information, quantum registers and quantum gates for data handling and processing and the development of quantum algorithms. In this talk, the basic functional principle of a quantum computer will be reviewed. It will be shown how strings of trapped ions can be used to build a quantum information processor and how basic computations can be performed using quantum techniques. In particular, the quantum way of doing computations will be illustrated by analog and digital quantum simulations and the basic scheme for quantum error correction will be introduced and discussed. Scaling-up the ion-trap quantum computer can be achieved with interfaces for ion-photon entanglement based on high-finesse optical cavities and cavity-QED protocols, which will be exemplified by recent experimental results.
Title: Quantum information and the monogamy of entanglement
Date/Time: 08-Dec, 04:00PM
Venue: Ngee Ann Kongsi Auditorium Level 2 Education Resource Centre University Town, NUS
Abstract: The recent field of quantum information and computing approaches quantum mechanics not as a source of paradoxes or difficulties, but as a new theory of information. For example, Heisenberg's uncertainty principle can be seen not only as limitation on our ability to measure, but also can be used to construct new methods of sending secret messages. In this talk, I will first give an overview of the mathematics of quantum information and computing. Then I'll discuss a phenomenon known as "monogamy of entanglement" that has been a recent focus of my own research. Entanglement is a quantum analogue to correlations from probability theory. Unlike correlations, however, entanglement cannot be shared without limit; i.e. it is monogamous. I will discuss the surprising implications of this fact for mathematics, physics and computer science.
Title: An atomic superfluid Bose-Einstein condensate in a ring
Date/Time: 07-Nov, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: We create an atomic-gas Bose-Einstein condensate in a ring-shaped trap and interrupt the circulation in the ring with a repulsive barrier. This system can exhibit behavior similar to that of a superconducting loop interrupted by a weak link or Josephson junction. We observe controllable, discrete phase slips (jumps in the phase winding number around the ring) and hysteretic behavior in such a system. A novel interference measurement can detect the presence, direction, and winding number of the circulation in the ring as well as provide a determination of the current-phase relationship of the weak link.
Title: Matter and Light: sharing ideas and concepts in the quantum world
Date/Time: 15-Oct, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: With the development of quantum engineering, the traditionally separated fields of condensed matter physics and quantum optics are exchanging ideas and concepts to their mutual benefit. In the first part of the talk, I will recap the development of electronic structure theory in materials from weak- to strong interactions and then discuss how ideas on strong correlations have penetrated into cold atomic physics and cavity optics in the last decade. In the second part I will discuss a reverse example, how ideas on entanglement, typically a realm of quantum optics, have started to penetrate into mesoscopic physics and define the new field of electronic optics.
Title: A single ion in a Penning Trap: Test of QED and a new value for the electron´s mass
Date/Time: 04-Sep, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The magnetic moment of the electron bound in hydrogen-like ions has been measured with high precision. We perform Zeeman-spectroscopy using single ions confined in a triple-Penning trap. The results are compared with QED calculations and represent to date the most precise QED test in bound systems. From a comparison of theory and experiment in C5+ where QED contributions are small and well known, we derive a value for the electron´s atomic mass which is more than one order of magnitude more precise than previously known. References: S. Sturm et al., g Factor of Hydrogen-like 28Si13+, Phys. Rev. Lett. 107, 023002 (2011) S. Sturm et a,, High precision measurement of the atomic mass of the electron, Nature 506, 467 (2014)
Title: A Mixture of Fermi and Bose Superfluids
Date/Time: 19-Aug, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Superconductivity and superfluidity are spectacular macroscopic manifestations of genuine quantum collective effects with, today, vast domains of applications. In this family of quantum solids or fluids, ultracold gases and polaritons are the last born. Thanks to the great flexibility of laser cooling and trapping methods, ultracold gases offer to study these quantum correlated systems with a new twist. It is possible for instance to tune the strength and sign of the interaction between atoms. Optical lattices, realized by interfering laser beams, create periodic optical potentials that mimic the crystalline potential seen by electrons in solids. In liquid helium and dilute gases, Bose and Fermi superfluidity has been observed separately, but producing a mixture where both the fermionic and the bosonic components are superfluid has been challenging. In this talk, we will describe the production of such a mixture of Bose and Fermi superfluids with dilute gases of two Lithium isotopes, 6Li and 7Li [1]. The mixture is remarkably stable and we probe the collective dynamics of this system by exciting center-of-mass oscillations that exhibit extremely low damping below a certain critical velocity. Using high precision spectroscopy of these modes we observe coherent energy exchange and measure the coupling between the two superfluids. Our observations can be captured theoretically using a sum-rule approach that we interpret in terms of two coupled oscillators. Tuning the attractive interaction in the Fermi superfluid, we measure the two speeds of sound in the mixture in the crossover between BEC of deeply bound dimers and BCS superfluidity. This provides a new method to probe the equation of state of the Fermi superfluid. [1] I. Ferrier-Barbut, M. Delehaye, S. Laurent, A. Grier, M. Pierce, B. Rem, F. Chevy, C. Salomon, ArXiv :1404.2548
Title: Quantum Photonics: Perspectives and challenges
Date/Time: 03-Jul, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I will review the status of quantum photonics focussing on the recent developments in spontaneous four wave mixing (FWM) in optical fibre1,2 and in silicon waveguides3. I will show that 4-photon and 6-photon experiments are now achievable with fibre sources but also that the scalability is primarily limited by loss. This is illustrated by recent 6-N00N interferometry demonstrations4 and a variety of cluster state based experiments5. In all experiments to date quantum advantage can be inferred within a detection post-selected sub-set of photons but this detection rate drops exponentially with increasing photon number. The integration of all components onto one chip could bring very large photon number experiments closer if waveguide, switching and detector losses can be made small. A future roadmap to making scalable quantum processors with linear optics resources is beginning to form but will require significant resource overhead. I will comment on emerging alternatives to linear optics approaches based on one dimensional cavities and waveguides that could reduce this overhead significantly6. [1] McMillan,et al Narrowband high-fidelity all-fibre source of heralded single photons at 1570 nm, Opt. Express 17, 6156-6165 (2009). [2] Halder, et al, Nonclassical 2-photon interference with separate intrinsically narrowband fibre sources, Opt. Express 17, 4670-4676 (2009). [3] Silverstone, et al, On-chip quantum interference between two silicon waveguide sources, Nature Photonics 8, 104 (2014). [4] Bell, et al, Multi-Colour Quantum Metrology with Entangled Photons, Phys. Rev. Letts. 111, 093603 (2013). [5] Bell, et al, Experimental characterization of universal one-way quantum computing, New J. Phys. 15, 053030 (2013). [6] Young et al, Polarization engineering in photonic crystal waveguides for spin-photon entanglers, arXiv 1406.0714
Title: Non-quantum Entanglement and Bell Violation Analogs
Date/Time: 13-Mar, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: We are interested in non-quantum entanglement in the context of probabilistic classical theories. We will examine a category of correlation measurements in statistical optics prompted by the following remark of John Bell: "It can indeed be shown that that the quantum mechanical correlations cannot be reproduced by a hidden variables theory even if one allows a 'local' sort of indeterminism. ... This would not work."
Title: Cats, Decoherence and Quantum Measurement
Date/Time: 27-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: In this talk it is our intention to review the basic ideas of how entanglement relates to the so-called Schrödinger cat state and present a paradigmatic situation where states very similar to that one can be created. The example we have chosen is the SQUID ring which depending on the external bias allows us to implement a wealth of interesting physical situations to be treated. We shall argue that in these situations the question of dissipation is really relevant and the concept of decoherence naturally arises. Once we have accomplished that we discuss some possible implications of decoherence to the quantum theory of measurement . As a matter of fact, we shall employ an alternative measure of quantum correlation which goes beyond entanglement – the quantum discord – with the same purpose. We finally present recent experimental results performed with twin photons which corroborate our predictions. This Colloquium is jointly organised with Graphene Research Centre, NUS
Title: Make It Small and Take It Outside!
Date/Time: 06-Dec, 06:00PM
Venue: Ngee Ann Auditorium, Asian Civilisations Museum, 1 Empress Place, Singapore 179555
Abstract: Quantum physics allows us to model the behavior of matter and energy at the atomic and molecular level (and even beyond!) with a very high precision. Unfortunately, the devices needed to control and observe these quantum effects are often large and unwieldy, making the deployment of quantum-inspired technology a serious challenge. At the Centre for Quantum Technologies there is a program called the Small Photon-Entangling Quantum System that seeks to build and deploy complete entangled photon generators/detectors that have a small physical footprint. These devices are designed to operate autonomously in remote nodes of quantum communication networks. An example would be nanosatellites in low earth orbit. I will present the key design parameters that have enabled us to reduce the resource requirements and will discuss the outlook for experiments and applications.
Title: Simulating Quantum Behaviour with Quantum Computers
Date/Time: 06-Dec, 05:00PM
Venue: Ngee Ann Auditorium, Asian Civilisations Museum, 1 Empress Place, Singapore 179555
Abstract: In 1982, Richard Feynman proposed the concept of a quantum computer as a means of simulating physical systems that evolve according to the Schrödinger equation. Since that time, a rich theory of quantum computing has developed, which includes quantum algorithms for simulating physical systems (as well as for several other computing problems). I will explain various quantum algorithms that have been proposed for Feynman's simulation problem, including my recent work (jointly with Dominic Berry and Rolando Somma) that dramatically improves the running time as a function of the precision of the output data.
Title: Superconducting Circuits for Quantum Information Processing: How Electric Circuits Behave Quantum Mechanically
Date/Time: 06-Dec, 04:00PM
Venue: Ngee Ann Auditorium, Asian Civilisations Museum, 1 Empress Place, Singapore 179555
Abstract: We all benefit from the amazing technologies of silicon large-scale integrated circuits (Si LSI) in our mobile phones, tablets, PCs, etc. It is almost incredible that billions of nanometer-scale transistors in a processor operate synchronously with a nanosecond clock cycle. Indeed, the emergence of the silicon empire in the last century is one of the biggest achievements of quantum mechanics and solid-state physics based on it. Nevertheless, circuit engineers are never bothered with quantum mechanics: While the dynamics of individual electrons can only be understood in the quantum languages, the operations of the devices are described fully classically, i.e., either ON or OFF. In contrast, superconducting circuits with a proper design can behave quantum mechanically in a macroscopic scale: Superposition of ON and OFF states is allowed in the quantum bit devices. I will discuss how to integrate such quantum bits and how to control and measure the quantum states toward realizing a superconducting quantum information processor.
Title: Uncertainty Principle and Quantum Reality
Date/Time: 28-Nov, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Abstract: In this talk, I will review my proposal to reformulate Heisenberg's uncertainty principle [1-3] and its recent experimental confirmation [6]. The new formulation allows us simultaneous measurements of totally non-commuting observables [4]. We will discuss how this affects our understanding of quantum reality along the line with a recent mathematical reconstruction of Bohr's reply to Einstein-Podolsky-Rosen on their paradox [5]. References. [1] M. Ozawa, Universally valid reformulation of the Heisenberg uncertainty principle on noise and disturbance in measurement, Phys. Rev. A 67, 042105/1-042105/6 (2003). [2] M. Ozawa, Uncertainty principle for quantum instruments and computing, Int. J. Quant. Inf. 1, 569--588 (2003). [3] M. Ozawa, Uncertainty relations for noise and disturbance in generalized quantum measurements, Ann. Phys. (N.Y.) 311, 350-416 (2004). [4] M. Ozawa, Quantum reality and measurement: A quantum logical approach, Found. Phys. 41, 592-607 (2011). [5] M. Ozawa and Y. Kitajima, Reconstructing Bohr's Reply to EPR in Algebraic Quantum Theory, Found. Phys. 42, 475-487 (2012). [6] J. Erhart, S. Sponar, G. Sulyok, G. Badurek, M. Ozawa, and Y. Hasegawa, Experimental demonstration of a universally valid error-disturbance uncertainty relation in spin-measurements, Nature Phys. 8, 185-189 (2012).
Title: Superoscillations and weak measurement
Date/Time: 03-Oct, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Band-limited functions can oscillate arbitrarily faster than their fastest Fourier component over arbitrarily long intervals. Where such ‘superoscillations’occur, functions are exponentially weak. In typical monochromatic optical fields, substantial fractions of the domain (one-third in two dimensions) are superoscillatory. Superoscillations have implications for signal processing, and raise the possibility of sub-wavelength resolution microscopy without evanescent waves. In quantum mechanics, superoscillations correspond to weak measurements, suggesting ‘weak values’ of observables (e.g photon momenta) far outside the range represented in the quantum state. A weak measurement of neutrino speed could lead to a superluminal result without violating causality, but the effect is too small to explain the speed claimed in a recent experminent.
Title: Energy Fluctuations and Maxwell Demon in Nano-electronic Circuits
Date/Time: 25-Jul, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: In small systems, such as molecules or nanostructures, energy fluctuations play an important role, and the second law of thermodynamics, for example, applies only on the average. The distribution of entropy production and the work performed under non-equilibrium conditions are then governed by so-called fluctuation relations [1 - 3]. I apply these concepts to a single-electron box [4,5], and present an experimental demonstration of fluctuation relations in them [6,7]. Single-electron circuits provide furthermore a basic example of a Maxwell Demon, where information can be converted into energy [8]; here the information is collected by a detector with single-electron sensitivity. Finally I discuss the subtle issues of work and heat in open quantum systems. I use superconducting qubits as examples of driven systems in this context [9,10]. [1] C. Jarzynski, Nonequilibrium equality for free energy differences, Phys. Rev. Lett. 78, 2690 (1997). [2] G. E. Crooks, Entropy production fluctuation theorem and the nonequilibrium work relation for free energy differences, Phys. Rev. E 60, 2721 (1999). [3] U. Seifert, Entropy Production along a Stochastic Trajectory and an Integral Fluctuation Theorem, Phys. Rev. Lett. 95, 040602 (2005). [4] D.V. Averin and J.P. Pekola, Statistics of the dissipated energy in driven single-electron transitions, EPL 96, 67004 (2011). [5] J. P. Pekola and O.-P. Saira, Work, Free Energy and Dissipation in Voltage Driven Single-Electron Transitions, J. Low Temp. Phys. 169, 70 (2012). [6] O.-P. Saira, Y. Yoon, T. Tanttu, M. Möttönen, D. V. Averin, and J. P. Pekola, Test of Jarzynski and Crooks fluctuation relations in an electronic system, Phys. Rev. Lett. 109, 180601 (2012). [7] J. V. Koski et al., Distribution of entropy production in nonequilibrium single-electron tunneling, arXiv:1303.6405. [8] D. V. Averin, M. Möttönen, and J. P. Pekola, Maxwell's demon based on a single-electron pump, Phys. Rev. B 84, 245448 (2011). [9] J. P. Pekola, P. Solinas, A. Shnirman, and D. V. Averin, Calorimetric measurement of quantum work, arXiv:1212.5808 (2012). [10] F. W. J. Hekking and J. P. Pekola, Quantum jump approach for work and dissipation in a two-level system, arXiv:1305.5207.
Title: Quoins Versus Coins
Date/Time: 18-Jul, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Human monkeys are used to thinking about the problem of choosing from a set of objects according to some desired, biased, probability distribution. Just think about how you chose your partner(s). Even when it is easy for you to do such a sampling (eg Monte Carlo sampling is sometimes easy in condensed matter physics), it can be difficult to do a quantum sampling (Q-Sampling) of the same distribution. By Q-Sampling I mean the creation of a coherent superposition of states of such objects whose amplitudes are the (square roots of) of the specified distribution. In this talk I will discuss what we do know about this problem, why it is interesting, and will discuss how it can let us achieve tasks with quantum information that are provably impossible classically. Unlike most (if not all) other such tasks in quantum information, this one does not have to do with communication per se.
Title: Quantum Computation of Prime Number Functions
Date/Time: 16-May, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: We propose a quantum circuit that creates a pure state corresponding to the quantum superposition of prime numbers. This {\em Prime} state can be built using an oracle which is a quantum implementation of the classical Miller-Rabin primality test. The {\em Prime} state is highly entangled, and its entanglement measures encode number theoretical functions such as the distribution of twin primes or the Chebyshev bias. This algorithm can be further combined with the quantum Fourier transform to yield an estimate of the prime counting function, more efficiently than any classical algorithm and with an error below the bound that allows for the verification of the Riemann hypothesis. Arithmetic properties of prime numbers are then, in principle, amenable to experimental verifications on quantum systems.
Title: Information thermodynamics and fluctuation theorems
Date/Time: 04-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The second law of thermodynamics presupposes a clear-cut distinction between the controllable and uncontrollable degrees of freedom by means of macroscopic operations. The cutting-edge technologies in quantum information and nanoscience seem to require us to abondon such a working hypothesis in favor of the distinction between the accessible and inaccessible degrees of freedom. In this talk, I will talk about the fundamentals of such information thermodynamics together with the related new results on fluctuation theorems
Title: New Lattice Gauge Theories From Quantum Computation
Date/Time: 21-Mar, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I present some views and perspectives on the notions of Fault-Tolerant Quantum Computation with topological codes Next, I present current results on how the process of external quantum error correction on topological color codes (TCCs) leads to new versions of LGTs (Lattice Gauge Theories). In particular, we find that: i/ A complete study of error correction in TCCs yields the error threshold of p_c = 4.5(2)%. ii/ A novel Abelian lattice gauge theory with gauge group Z_2xZ_2 and a peculiar lattice and gauge structure that departs from the standard formulations of Wegner and Wilson. We refer to it as a tricolored LGT. Its structure reflects the error history in color codes, rather than the discretization of a continuous gauge theory. iii/ A novel approach to pinpoint first-order phase transitions in LGTs with disorder using the skewness of the average over Wilson loop operators. Finally, we show how to increase the error threshold up to 18.9(3)% when noise correlations are taken into account in depolarizing channels
Title: A high-energy particle experiment on a tabletop: what you can learn at 100 meV that you can't learn at 100 TeV
Date/Time: 23-Jan, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Any time anyone has ever measured, the north and south poles of an electron are seen to be identical. But there are good theoretical reasons to suspect that upon closer examination we may find a discrepancy. Observing such a discrepancy would be the equivalent recording a spark chamber track of a never before seen supersymmetric particle. We will take advantage of nature's high-electric-field laboratory -- a polar molecule -- to reach new levels of sensitivity to the electon's hypothetical electric dipole moment.
Title: Quantum Technology for a Networked World
Date/Time: 07-Dec, 06:00PM
Venue: NUS Kent Ridge, University Hall Auditorium, Level 2, Lee Kong Chian Wing
Abstract: The 21st Century has seen the emergence of a networked world, connected by global fibre-optic communications and mobile phones, with geo-location provided through GPS, and all this has changed our lives more dramatically than at any time since the industrial revolution. Quantum-enabled technology is at the heart of this change. I will describe how communications depend on lasers that are shot noise limited, geo-located and synchronized by atomic clocks that depend on atomic coherence. New developments in quantum technology, and in particular miniature atomic clocks that utilize concepts such as coherent population trapping have the potential for even more dramatic applications. Some of these include comms systems immune to GPS jamming (of real importance for global security), as well as quantum sensors for medical applications (including cardiography, neurophysiology, etc), sensitive magnetometry, gyros, and geophysical surveying. I will describe the basic quantum phenomena being exploited as well as prospects for exploitation.
Title: Randomness
Date/Time: 07-Dec, 04:00PM
Venue: NUS Kent Ridge, University Hall Auditorium, Level 2, Lee Kong Chian Wing
Abstract: Is the universe inherently deterministic or probabilistic? Perhaps more importantly - can we tell the difference between the two? Humanity has pondered the meaning and utility of randomness for millennia. There is a remarkable variety of ways in which we utilize perfect coin tosses to our advantage: in statistics, cryptography, game theory, algorithms, gambling… Indeed, randomness seems indispensable! Which of these applications survive if the universe had no randomness in it at all? Which of them survive if only poor quality randomness is available, e.g. that arises from "unpredictable" phenomena like the weather or the stock market? A computational theory of randomness, developed in the past three decades, reveals (perhaps counter-intuitively) that very little is lost in such deterministic or weakly random worlds. In the talk I'll explain the main ideas and results of this theory.
Title: Fluctuoscopy of Superconductors and Dynamics of Abrikosov’s Lattice Formation Close to Hc2(0)
Date/Time: 29-Nov, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I start my lecture from qualitative discussion, based on the Heisenberg principle, of the nature of thermal fluctuations in superconductor at temperatures above the critical one. Then the analogous consideration is applied to the regime of quantum fluctuations at zero temperature above the field Hc2(0). Basing both on microscopic and qualitative analysis we demonstrate, that here, fluctuating Cooper pairs rotating in magnetic field present themselves precursor images of Abrikosov’s vortices and form the clusters with specific superconducting features. I evaluate both the characteristic size QF( H) and lifetime QF( H) of such formations. When magnetic field reaches Hc2(0) from above the size and lifetime of such clusters tend infinity and the order, corresponding Abrikosov’s lattice is established. In second part of the lecture I discuss fluctuoscopy - the method of investigation of intrinsic properties of superconductors by means of the detailed analysis of their fluctuation magneto-conductivity, tunneling characteristics, Nernst coefficient throughout the phase diagram.
Title: Single-atom spin qubits in silicon
Date/Time: 25-Oct, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The idea of using the spin of a single donor atom in silicon to encode quantum information goes back to the Kane proposal [1] in 1998. The proposal was motivated by two observations: (i) Silicon is one of the most promising materials to host spin qubits in solid state, owing to the very weak spin-orbit coupling, and to the possibility to eliminate decoherence from nuclear spins by isotopic purification; (ii) A trillion-dollar worth industry already exists, that has developed extraordinary tools to manufacture silicon nanoscale devices in a reliable and cost-effective way. The proposal appeared ambitious, visionary but very challenging at the time, because it relied upon the non-trivial assumption that the progress in the fabrication of classical silicon devices could be harnessed to pursue quantum information goals. Indeed, over a decade of intense efforts has been necessary before the first breakthroughs in silicon quantum technologies could be demonstrated. I will present the first experimental demonstration of a qubit based on a single phosphorus atom in silicon. The atom is coupled to a silicon Single-Electron Transistor, and the whole device is fabricated retaining standard CMOS technologies such as ion implantation [2] and metal gates fabricated on top of high-quality silicon oxide [3]. In a single-atom device, we have demonstrated single-shot readout [4] and coherent control [5] of the donor electron spin, as well and the spins of the 31P nucleus and of a strongly-coupled 29Si nucleus. All three qubits exhibit excellent coherence and high-fidelity readability, with the nuclear ones being accessible through a quantum nondemolition measurement. These results represent major milestones in the search for a scalable and coherent quantum computer platform, and confirm the vision of silicon as the choice material for both quantum and classical technologies. [1] B. E. Kane, Nature 393, 133 (1998). [2] D. N. Jamieson et al., Appl. Phys. Lett. 86, 202101 (2005). [3] A. Morello et al., Phys. Rev. B 80, 081307(R) (2009). [4] A. Morello et al., Nature 467, 687 (2010). [5] J. J. Pla et al., Nature (2012), in press.
Title: Generating and exploiting intense attosecond pulses
Date/Time: 06-Sep, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: During the last decade we have systematically developed methods and instrumentation leading to the coherent emission of intense XUV pulses of sub-femtosecond duration. As a measure of the term "intense" we identify the feasibility in inducing observable non-linear processes solely by the XUV radiation. Non-linear XUV processes are essential in (a) attosecond pulse metrology, (b) XUV-pump-XUV-probe studies of ultrafast dynamics, (c) reaching highest spatiotemporal resolution and d) exploring new physics in inner-shell non-linear or even strong field processes. We regard the above as current and future main steam developments in attoscience. I will review the physics and technology behind intense attosecond pulse train [1,2] and coherent XUV supercontinuum [3] generation, as well as recent XUV-pump-XUV-probe studies at the boundary between fempto- and atto-second scales [4]. 1.P. Tzallas, D. Charalambidis, N.A. Papadogiannis, K. Witte and G. D. Tsakiris Nature 426, 267 (2003) 2.Y. Nomura, R. Hörlein, P. Tzallas, B. Dromey, S. Rykovanov, S. Major, J. Osterhoff, S. Karsch, M. Zepf, D. Charalambidis, F. Krausz, G.D. Tsakiris Nature Physics 5, 124 (2009) 3.P. Tzallas, E. Skantzakis, C. Kalpouzos, E. Benis, G. D. Tsakiris and D. Charalambidis Nature Physics 3, 846 (2007) 4.P. Tzallas, E. Skantzakis, L. A. A. Nikolopoulos, G. D. Tsakiris and D. Charalambidis Nature Physics 7, 781 (2011)
Title: The evasive cheshire cat: How to detect fractional statistics
Date/Time: 02-Aug, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Certain strongly correlated electronic systems give rise to quantized low energy excitations which possess fractional statistics. A number of protocols to detect such quasi particles—anyons-- have been proposed in recent years, yet the quest for fractional statistics has not resulted in an unequivocal manifestation. I will review some of theoretical proposals for detection of anyons, and discuss why it is so difficult to find them
Title: Doing small systems: Fluctuation relations and the arrow of time
Date/Time: 26-Jul, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: This talk is aimed at highlighting issues that relate of doing thermodynamics and statistical physics of finite size systems. This theme gained considerable importance in view of fascinating advances in nanotechnology and system biology. While the fathers of thermodynamics developed the famous 5 Laws of Thermodynamics (incl. a “–1_Law), having in mind macroscopic systems, these grand concepts need to be inspected anew in view of the fact that the fluctuations grow with decreasing size to a level where they even may play the dominant role. This holds true particularly for the inter-relationships between fluctuations and, importantly, the measurements of work, heat & heat flow and thermodynamic equilibrium quantifiers such as (non-fluctuating) free energy changes or production of thermodynamic entropy. -- Subtleties occur in equilibrium thermodynamics for small systems, such as the role of finite size for quantities like (possibly negative-valued) canonical heat capacitance in presence of strong system-environment coupling. I further elaborate on recent, timely results for nonequilibrium classical and quantum fluctuation relations. An attempt is made to outline pitfalls and still open issues (relativistic and others) together with those inherent difficulties that likely do emerge with the experimental validation scenarios of such relations. Finally, a connection between Fluctuation Relations, nonlinear Response and its feasible relation with the ever-lasting intriguing challenge in detecting the origin of an "Arrow of Time" will be elucidated. This presentation is based on joint work with Michele Campisi, Gert-Ludwig Ingold and Peter Talkner, all at the University of Augsburg. -- Own pertinent recent works that apply for the theme of this talk are: [1] G. L. Ingold, P. Hänggi, and P. Talkner Specific heat anomalies of open quantum systems Phys. Rev. E 79, 061105 (2009) [2] M. Campisi, P. Hanggi, and P. Talkner Colloquium: Quantum fluctuation relations: Foundations and applications Rev. Mod. Phys. 83, 771--791 (2011); Addendum and Erratum: Quantum fluctuation relations: Foundations and applications Rev. Mod. Phys. 83, 1653 (2011). [3] M. Campisi and P. Hanggi Fluctuation, Dissipation and the Arrow of Time Entropy 13, 2024--2035 (2012).
Title: Simulating quantum transport with atoms and light
Date/Time: 24-May, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The transport of quantum particles in non ideal material media (eg the conduction of electrons in an imperfect crystal) is strongly affected by scattering from impurities of the medium. Even for a weak disorder, semi-classical theories, such as those based on the Boltzmann equation for matter-waves scattering from the impurities, often fail to describe transport properties and full quantum approaches are necessary. The properties of the quantum systems are of fundamental interest as they show intriguing and non-intuitive phenomena that are not yet fully understood such as Anderson localization, percolation, disorder-driven quantum phase transitions and the corresponding Bose-glass or spin-glass phases. Understanding quantum transport in amorphous solids is one of the main issues in this context, related to electric and thermal conductivities. Ultracold atomic gases can now be considered to revisit the problem of quantum conductivity and quantum transport under unique control possibilities. Dilute atomic Bose-Einstein condensates (BEC) and degenerate Fermi gases (DFG) are produced routinely taking advantage of the recent progress in cooling and trapping of neutral atoms. Transport has been widely investigated in controlled potentials with no defects, for instance periodic potentials (optical lattices). Controlled disordered potentials can also be produced with various techniques such as the use of magnetic traps designed on atomic chips with rough wires, the use of localized impurity atoms, the use of radio-frequency fields or the use of optical potentials. This recently lead to the observation of the Anderson Localization of a BEC in 1D and 3D,and the study of diffusion properties during matter-wave transport.
Title: Breaking the bounds of quantum thermodynamics
Date/Time: 08-Mar, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I shall revisit the traditional formulation of the three laws of thermodynamics and the bounds they imply on thermodynamic observables , and argue that all of these have to be modified and reformulated when the system is enacted upon on time scales shorter than the bath memory time. Practical consequences related to work extraction and cooling will be demonstrated to be in stark contrast with existing schemes .
Title: Carbon Spintronics
Date/Time: 09-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Carbon, in the form of graphene and carbon nanotubes, has recently emerged as an interesting alternative material for electronics. Here, we argue that carbon is also a unique material for a new type of electronics that is based on the electron spin rather than its charge, known as spintronics, and in particular for spin-based quantum computing [1]. Due to the low concentration of nuclear spins and relatively weak spin-orbit coupling, carbon-based structures allow for long coherence times, which is the primary figure of merit for the quality of a spin quantum bit (qubit). We discuss the formation of quantum dots, acting as electron “traps’’, in graphene and their potential use for quantum information processing. In diamond, the spin coherence of defect centers can persist even at ambient temperatures. After introducing this fascinating quantum system, we briefly present a particular mechanism for storing and retrieving quantum information in an atomic nucleus in diamond [2]. [1] B. Trauzettel, D. Bulaev, D. Loss, and G. Burkard, Nature Phys. 3, 192 (2007). [2] G. D. Fuchs, G. Burkard, P. V. Klimov, and D. D. Awschalom, Nature Phys. 7, 789 (2011).
Title: Experimental Quantum Error Correction
Date/Time: 12-Jan, 05:30PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The Achilles' heel of quantum information processors is the fragility of quantum states and processes. Without a method to control imperfection and imprecision of quantum devices, the probability that a quantum computation succeed will decrease exponentially in the number of gates it requires. In the last fifteen years, building on the discovery of quantum error correction, accuracy threshold theorems were proved showing that error can be controlled using a reasonable amount of resources as long as the error rate is smaller than a certain threshold. We thus have a scalable theory describing how to control quantum systems. I will briefly review some of the assumptions of the accuracy threshold theorems and comment on recent experiments that have been done and should be done to turn quantum error correction into an experimental reality.
Title: Quantum Theory of the Classical
Date/Time: 12-Jan, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I discuss three insights into the transition from quantum to classical. I will start with (i) a minimalist (decoherence-free) derivation of preferred states. Such pointer states define events (e.g., measurement outcomes) without appealing to Born's rule. Probabilities and (ii) Born’s rule can be derived from the symmetries of entangled quantum states. With probabilities at hand one can analyze information flows from the system to the environment in course of decoherence. They explain how (iii) robust “classical reality” arises from the quantum substrate by accounting for objective existence of pointer states of quantum systems through redundancy of their records in the environment. Taken together, and in the right order, these three advances elucidate quantum origins of the classical.* *W. H. Zurek, Nature Physics 5, 181-188 (2009).
Title: Controlling and Exploring Quantum Gases at the Single Atom Level
Date/Time: 07-Dec, 04:00PM
Venue: NUS University Hall Auditorium, Lee Kong Chian Wing, Level 2
Abstract not available.
Title: Position-based Cryptography
Date/Time: 07-Dec, 04:00PM
Venue: NUS Kent Ridge, University Hall Auditorium, Level 2, Lee Kong Chian Wing
Abstract not available.
Title: Quantum flows in polariton condensates
Date/Time: 24-Nov, 12:00AM
Venue: CQT Seminar Room, S15-03-15
Abstract: Superfluidity, the ability of a quantum fluid to flow without friction, is one of the most spectacular phenomena occurring in degenerate gases of interacting bosons. Since its first discovery in liquid helium-4, superfluidity has been observed in quite different systems, and recent experiments with ultra-cold trapped atoms have explored the subtle links between superfluidity and Bose–Einstein condensation. In semiconductor microcavities, exciton-polaritons, which are mixed light-matter quasi-particles arising from the strong coupling between photons and excitons, have been shown to form Bose-Einstein condensates. It has been predicted that polaritons should behave as a novel quantum fluid, with unique properties stemming from their intrinsically non-equilibrium nature and their very low mass (~10-4 times that of the electron, inherited from their photonic component). This has stimulated the quest for an experimental demonstration of superfluidity effects in polariton systems. I will present our recent results, demonstrating superfluid motion of polaritons, which manifests itself as the suppression of scattering from defects when the flow velocity is slower than the speed of sound in the fluid. Cerenkov-like wake patterns, vortices and dark solitons are also observed when the flow velocity is varied. The experimental findings are in quantitative agreement with predictions based on a generalized Gross–Pitaevskii theory, and establish microcavity polaritons as a system for exploring the rich physics of non-equilibrium quantum fluids.
Title: Quantum Information Processing and Chemistry
Date/Time: 06-Oct, 12:00AM
Venue: CQT Seminar Room, S15-03-15
Abstract: In this talk, I overview some of the aspects that intersect quantum information science and problems in chemistry. In particular, I will describe the simulation of chemical dynamics and electronic structure using quantum computers, both algorithms and experimental implementations. I will discuss the use of quantum adiabatic or annealing devices for solving lattice heteropolymer models associated with protein folding.
Title: Spinor- and Rydberg- Polaritons
Date/Time: 11-Aug, 12:00AM
Venue: CQT Seminar Room, S15-03-15
Abstract: Slow-light polaritons are quasi-particles generated in the interaction of light with multi-level atoms driven by an external laser close to a Raman resonance. Their dispersion relation can be controlled to a large extend, representing massive Schroedinger particles on the one hand or multi-component, i.e. spinor objects with a Dirac-like spectrum on the other. In the latter case "relativistic" length and energy scales can be widely tuned, making relativistic effects accessible in the lab. Making use of the tunability of the mass the delocalization transition of the random-mass Dirac model with off-diagonal disorder can be experimentally observed. In the second part of the talk the prospects to create strong interactions between dark-state polaritons using Rydberg atoms will be discussed. The dipole-dipole coupling between atoms in a Rydberg state leads to a strong and long-range interaction between polaritons, as well as to a blockade phenomenon. This interacion can give rize to interesting many-body phenomena, such as two-particle correlations which are much stronger than possible for pointlike interacting particles, crystallization of photons or quantum Hall states.
Title: Anderson Localization – looking forward.
Date/Time: 28-Jul, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Localization of the eigenfunctions of quantum particles in a random potential was discovered by P.W. Anderson more than 50 years ago. In spite of its respectable maturity and intensive theoretical and experimental studies this field is far from being exhausted. Anderson localization was originally discovered in connection with spin relaxation and charge transport in disordered conductors. Later this phenomenon was observed for light, microwaves, sound, and more recently for cold atoms. Moreover, it became clear that the domain of applicability of the concept of localization is much broader. For example, it provides an adequate framework for discussing the transition between integrable and chaotic behavior in quantum systems. We will discuss current understanding of the Anderson localization and its manifestation in different physical situations. We will illustrate the main idea by several examples from adiabatic quantum computation to many-body statistical mechanics. We will demonstrate that physics of disordered many-body quantum systems can be described in the framework of the Anderson Localization.
Title: Is smell a quantum sense ?
Date/Time: 26-May, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Our sense of smell is a extraordinarily good at molecular recognition: we can identify tens of thousands of odorants unerringly over a wide concentration range. The mechanism by which this happens do so is still hotly debated. One view is that molecular shape governs smell, but this notion has turned out to have very little predictive power. Some years ago I revived a discredited theory that posits instead that the nose is a vibrational spectroscope, and proposed a possible underlying mechanism, inelastic electron tunneling. In my talk I will review the history and salient facts of this problem and describe some recent experiments that go some way towards settling the question.
Title: Dipole-Dipole Interactions in the Frozen Rydberg Gas
Date/Time: 24-Mar, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Experiments probing the dipole-dipole interactions in a frozen Rydberg gas made from 300μK Rb in a magneto optical trap are described. Resonant energy transfer measurements, in which the energy of an initial pair of atoms is tuned into resonance with the energy of a final state pair exhibit widths in excess of the simplest binary interaction estimates. We attribute the larger than expected widths to the existence of many body interactions and the fact that since the detection favors close pairs of atoms. In contrast, recent measurements of the dipole-dipole broadening of ns-np microwave transitions show less broadening than the simplest estimates and cusped lineshapes. These observations can be understood by considering several factors; the reduction, even to zero, of the dipole-dipole energy shifts due to the spin-orbit interaction, the strengths of the allowed microwave transitions, and the fact that many of the atoms are in low density regions of the trap. While the gas is frozen on a 1μs time scale, the attractive force associated with the dipole-dipole interaction leads to ionizing collisions on a 5-10 μs time scale, which can in turn lead to the spontaneous evolution to a plasma. There are, however, open questions related to the evolution to a plasma.
Title: Integrated Quantum Photonics
Date/Time: 10-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Quantum information science aims to harness uniquely quantum mechanical properties to enhance measurement and information technologies, and to explore fundamental aspects of quantum physics. Of the various approaches to quantum computing [1], photons are particularly appealing for their low-noise properties and ease of manipulation at the single qubit level [2]. Encoding quantum information in photons is also an appealing approach to quantum communication, metrology (eg. [3]), measurement (eg. [4]) and other quantum technologies [5]. However, the implementation of optical quantum circuits with bulk optics has reached practical limits. We have developed an integrated waveguide approach to photonic quantum circuits for high performance, miniaturisation and scalability [6]. Here we report high-fidelity silica-on-silicon integrated optical realisations of key quantum photonic circuits, including two-photon quantum interference and a controlled-NOT logic gate [7]. We have demonstrated controlled manipulation of up to four photons on-chip, including high-fidelity single qubit operations, using a lithographically patterned resistive phase shifter [8]. We have used this architecture to implement a small-scale compiled version of Shor's quantum factoring algorithm [9] and demonstrated heralded generation of tuneable four photon entangled states from a six photon input [10]. We have combined waveguide photonic circuits with superconducting single photon detectors [11]. We describe complex quantum interference behaviour in multi-mode interference devices with up to eight inputs and outputs [12], and quantum walks of correlated particles in arrays of coupled waveguides [13]. Finally, we give an overview of our recent work on fundamental aspects of quantum measurement [14,15] and single photon sources [16,17]. [1] T. D. Ladd, F. Jelezko, R. Laflamme, Y. Nakamura, C. Monroe, and J. L. OBrien, Nature 464, 45 (2010). [2] J. L. O'Brien, Science 318, 1567 (2007). [3] T. Nagata, R. Okamoto, J. L. O'Brien, K. Sasaki, and S. Takeuchi, Science 316, 726 (2007). [4] R. Okamoto, J. L. O'Brien, H. F. Hofmann, T. Nagata, K. Sasaki, and S. Takeuchi, Science 323, 483 (2009). [5] J.L.O'Brien, A.Furusawa, and J.Vuckovic, Nature Photon. 3, 687 (2009). [6] A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, and J. L. O'Brien, Science 320, 646 (2008). [7] A. Laing, A. Peruzzo, A. Politi, M. R. Verde, M. Halder, T. C. Ralph, M. G. Thompson, and J. L. O'Brien, Appl. Phys. Lett. 97, 211109 (2010) [8] J. C. F. Matthews, A. Politi, A. Stefanov, and J. L. O'Brien, Nature Photon. 3, 346 (2009). [9] A. Politi, J. C. F. Matthews, and J. L. O'Brien, Science 325, 1221 (2009). [10] J. C. F. Matthews, A. Peruzzo, D. Bonneau, and J. L. O'Brien, arXiv:1005.5119 [11] C. M. Natarajan, A. Peruzzo, S. Miki, M. Sasaki, Z. Wang, B. Baek, S. Nam, R. H. Hadfield, and J. L. O'Brien, Appl. Phys. Lett. 96, 211101 (2010). [12] A. Peruzzo, A. Laing, A. Politi, T. Rudolph, and J. L. O'Brien, Nature Comm. in press; arXiv:1005.5119 [13] A. Peruzzo, M. Lobino, J. C. F. Matthews, N. Matsuda, A. Politi, K. Poulios, X.-Q. Zhou, Y. Lahini, N. Ismail, K. Worhoff, Y. Bromberg, Y. Silberberg, M. G. Thompson, and J. L. O'Brien, Science 329, 1500 (2009) [14] A. Laing, T. Rudolph, and J. L. O'Brien, Phys. Rev. Lett. 102, 160502 (2009). [15] X-Q Zhou, TC Ralph, P Kalasuwan, M Zhang, A Peruzzo, BP Lanyon, and JL OBrien arXiv:1006.2670 [16] J. P. Hadden, J. P. Harrison, A. C. Stanley-Clarke, L. Marseglia, Y.-L. D. Ho, B. R. Patton, J. L. OBrien, and J. G. Rarity, Appl. Phys. Lett. 97, 241901 (2010) [17] C. Xiong, G. D. Marshall, A. Peruzzo, M. Lobino, A. S. Clark, D.-Y. Choi, S. J. Madden, C. M. Natarajan, M. G. Tanner, R. H. Hadfield, S. N. Dorenbos, T. Zijlstra, V. Zwiller, M. G. Thompson, J. G. Rarity, M. J. Steel, B. Luther-Davies, B. J. Eggleton, J. L. O’Brien arXiv:1011.1688
Title: Random numbers certified by Bell’s Theorem
Date/Time: 09-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Randomness is an intriguing concept which has fascinated, and keeps fascinating, many different communities, from Philosophy to Physics and Mathematics. On the other hand, randomness has also become a useful resource, as random numbers are used, for instance, in cryptographic applications, gambling or the simulation of physical and biological systems. Up to now, any of the existing solutions for randomness generation had to face the following problems: (i) certification: how can one prove that the obtained symbols are truly random? (ii) privacy: how can one be sure that the generated symbols are also random to any other external observer and (iii) device-independence: how do imperfections in the devices used in the generation process affect the randomness of the generated symbols? We provide a novel formalism for randomness generation which solves all these problems: using the non-local correlations of entangled quantum states, it is possible to generate certifiable, private and device-independent randomness.
Title: Information is Quantum: How physics has helped us understand what information is and what can be done with it.
Date/Time: 11-Jan, 05:00PM
Venue: Ngee Ann Kongsi Auditorium (Level 2), Singapore Management University
Abstract: The information revolution is largely based on what a physicist would call a classical view of information, assuming that it can be copied freely and is not disturbed by observation. Quantum effects in information processing, which prevent the information in microscopic objects like atoms or photons from being observed or copied accurately, were long regarded as a mere nuisance, but are now known to make possible feats such as quantum cryptography, quantum teleportation and dramatic computational speedups. Although progress toward a practical quantum computer is slow, other surprising quantum informational effects continue to be discovered, and quantum cryptographic systems are already available commercially. Most importantly, the quantum approach has led to a more coherent and powerful way of thinking about how physical objects interact and influence one another, and how that interaction can be used to compute, communicate, and protect privacy. This talk will avoid mathematical complications and instead aim to explain central quantum concepts like entanglement, which at first sight seem counterintuitive.
Title: CQT Annual Symposium - The Famous, The Bit and The Quantum
Date/Time: 07-Dec, 12:00AM
Venue: NUS University Hall Auditorium, Lee Kong Chian Wing, Level 2
Abstract not available.
Title: The past of a quantum particle in our many-worlds universe
Date/Time: 07-Oct, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I will argue that if we want to believe that physics of today can explain everything, we have to accept existence of parallel worlds in our Universe. I will explain how to see an object without light and why an attempt prove, without light, that an object is absent, fails. This will demonstrate that Wheeler's approach to the past of a quantum particles does not explain the weak trace it leaves and that the proper description of the past of a quantum particles requires addition of a quantum state evolving backward in time.
Title: Where is Quantum Mechanics Likely to Break Down?
Date/Time: 09-Sep, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Quantum theory is very robust and nobody knows where it will break down. However, there are certain structural weaknesses that offer clues as to where it may ultimately falter. One is its total disconnect to gravity at a very fundamental level (a physical level, not due to mathematical problems like non-linearity, etc). Even at the level of the equivalency principle there is a basic incompatibility, which can be traced back to the lack of a fundamental length scale (independent of the Planck length, which is important but not relevant here). We believe such a scale exists and will be ultimately responsible for the breakdown of the theory as we know it today.
Title: Quantum-limited measurements: One physicist's crooked path from quantum optics to quantum information
Date/Time: 20-Aug, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: We will first review the recent progress in atomic clocks operating in the microwave and optical domain of the electromagnetic spectrum. The second of the SI system of units is realized today with an accuracy of 3 10-16 by a number of laser cooled atomic fountains worldwide. Optical clocks have recently reached a frequency stability and accuracy in the 10-18 range [1] opening new perspectives for time keeping and fundamental tests. We will then present the status of the ACES mission of the European Space Agency scheduled for flight to the International Space Station from 2013 to 2015 [2]. ACES will embark a laser cooled cesium clock designed for microgravity operation (PHARAO ), an active hydrogen maser (SHM), and a high precision time transfer system operating in the microwave domain. This microwave link (MWL) will enable frequency comparisons between the space clocks and a network of ground based clocks belonging to worldwide metrology institutes and universities. The link is designed for obtaining a relative frequency resolution of 10-17 after a few days of measurement duration for intercontinental comparisons. In 2009-2010, all elements of the flight payload have successfully passed the Engineering Model tests and flight models are under construction. We will present the latest measurement results and flight model designs. In a second part we will describe tests of fundamental physical laws using ultra-stable clocks in space and on the ground that are planned for the ACES mission. An improved measurement of Einstein's gravitational red-shift will be made at the two parts per million level. By comparing clocks of different nature at the 10-17/year level, new limits will be obtained for the time variation of the fundamental constants of physics such as the fine structure constant alpha and the ratio of electron to proton mass. The ability to compare microwave and optical clocks using the recently developed frequency comb technique opens a wide range of possibilities in clock comparisons. Finally a new kind of relativistic geodesy based on the Einstein effect will provide information on the Earth geoid, complementing the recent determination obtained by space geodesy methods. References: [1]C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband, Phys. Rev. Lett. 104, 070802 (2010) [2] L. Cacciapuotti, and C. Salomon, Eur. Phys. J. Special Topics, 172, 57 (2009)
Title: Space Clocks and Fundamental Tests
Date/Time: 10-Aug, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: We will first review the recent progress in atomic clocks operating in the microwave and optical domain of the electromagnetic spectrum. The second of the SI system of units is realized today with an accuracy of 3 10-16 by a number of laser cooled atomic fountains worldwide. Optical clocks have recently reached a frequency stability and accuracy in the 10-18 range [1] opening new perspectives for time keeping and fundamental tests. We will then present the status of the ACES mission of the European Space Agency scheduled for flight to the International Space Station from 2013 to 2015 [2]. ACES will embark a laser cooled cesium clock designed for microgravity operation (PHARAO ), an active hydrogen maser (SHM), and a high precision time transfer system operating in the microwave domain. This microwave link (MWL) will enable frequency comparisons between the space clocks and a network of ground based clocks belonging to worldwide metrology institutes and universities. The link is designed for obtaining a relative frequency resolution of 10-17 after a few days of measurement duration for intercontinental comparisons. In 2009-2010, all elements of the flight payload have successfully passed the Engineering Model tests and flight models are under construction. We will present the latest measurement results and flight model designs. In a second part we will describe tests of fundamental physical laws using ultra-stable clocks in space and on the ground that are planned for the ACES mission. An improved measurement of Einstein's gravitational red-shift will be made at the two parts per million level. By comparing clocks of different nature at the 10-17/year level, new limits will be obtained for the time variation of the fundamental constants of physics such as the fine structure constant alpha and the ratio of electron to proton mass. The ability to compare microwave and optical clocks using the recently developed frequency comb technique opens a wide range of possibilities in clock comparisons. Finally a new kind of relativistic geodesy based on the Einstein effect will provide information on the Earth geoid, complementing the recent determination obtained by space geodesy methods. References: [1]C. W. Chou, D. B. Hume, J. C. J. Koelemeij, D. J. Wineland, and T. Rosenband, Phys. Rev. Lett. 104, 070802 (2010) [2] L. Cacciapuotti, and C. Salomon, Eur. Phys. J. Special Topics, 172, 57 (2009)
Title: Computing with Quantum Knots: Majorana Fermions, Non-Abelian Anyons, and Topological Quantum Computation
Date/Time: 27-May, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I will discuss the revolutionary new concept of topological quantum computation, which is fault-tolerant at the hardware level with no need, in principle, of any quantum error correction protocols. Errors simply do not happen since the physical qubits and the computation steps are protected against decoherence by non-local topological correlations in the underlying physical system. The key idea is non-Abelian statistics of the quasiparticles (called 'anyons' as opposed to fermions or bosons), where the space-time braiding of the anyons around each other, i.e. quantum 'knots', form topologically protected quantum gate operations. I will describe in details the status of the subject by discussing the theoretical principles guiding the experimental search for the appropriate topological phases of matter where such non-Abelian anyons may exist. Among the most significant possibilities are certain even-denominator fractional quantum Hall states, exotic chiral p-wavesuperconductors, sandwich structures made from superconductors/semiconductors or superconductors/insulators, and suitable cold atomic systems. In the context, I will also discuss the race to find Majorana fermions in solid state systems, with the Majorana fermions being the simplest generic examples of non-Abelian objects in nature. I will explain how the subject of topological quantum computation synergistically brings together conformal field theory and advanced mathematics on one hand with materials science and quantum information on the other
Title: Quantum Optics in Wavelength Scale Structures
Date/Time: 22-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The interaction between light and matter in optical structures that are at or below the wavelength scale could provide unprecedented performance in the storage of data, the switching of light and the generation of light of tailored properties. For instance wavelength scale confinement can enhance non-linear effects. An example is our experiments on four wave mixing effects in micron diameter optical fibres which provide novel sources of entangled photon pairs. Also by creating nanoscale light traps, cavities, in wavelength scale periodic dielectric structures we can drastically modify light emission from 'atom like' objects (quantum dots, colour centres). In the case of long light-storage times we can achieve strong coupling which is a form of entanglement between the atom-cavity system and single photons. When we couple light in and out of these structures we will see strong non-linear effects down to the single photon level. Thus we could make a low power switch at light levels orders of magnitude below existing technology. We can also expect to exploit the quantum properties as ultimately we have the single photon non-linear element needed for quantum computation. I will show a few example results to illustrate these ideas and discuss future plans now funded through an ERC advanced grant.
Title: One World Versus Many
Date/Time: 18-Mar, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: There is a compelling intellectual case for exploring whether purely unitary quantum theory defines a sensible and scientifically adequate theory, as Everett originally proposed. Many different and incompatible attempts to define a coherent Everettian quantum theory have been made over the past fifty years, suggesting this is a problem about which humans are excellent at forming strong intuitions but very bad at forming persuasive arguments. In this talk I review recent work in this area. I argue that considerable light is shed on the problem once one realizes that many-worlds theories are just that -- novel and distinct scientific theories, not reinterpretations of standard quantum theory. This forces us to reconsider from first principles whether (and if so how) we can relate many-worlds theories to empirical data. I review some interesting and ingenious attempts in this direction by Wallace, Greaves-Myrvold and others, and explain why they don't work.
Title: Ensemble encoding of quantum registers for quantum computing and communication
Date/Time: 24-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: In the conventional lay-out of a register for quantum computing, all qubits are associated with individual two-level quantum systems. Individual addressing and interaction with these systems permit one-bit gates, while a controllable pair-wise interaction between systems is needed to accomplish two-bit gates. We present a different approach that encodes a multi-bit quantum register in the collective internal state populations of an ensemble of identical multi-level quantum systems. This method establishes a linear relationship between the number of bits and the internal state Hilbert space dimension. It does not require experimental access to individual particles, but it relies on an interaction that restricts the collective populations to the (bit-) values zero and unity. We devote a detailed discussion to applications with neutral atoms interacting via Rydberg excited states, and we show that, e.g., a small cloud of cesium atoms may be operated as a universal quantum computer with up to 14 bits and as a deterministic multi-mode photonic device. Other schemes for collective encoding of quantum registers in hybrid quantum systems will be briefly discussed.
Title: Quantum Information: from burlesque ideas to a new technology
Date/Time: 08-Dec, 06:00PM
Venue: NUS University Hall Auditorium, Lee Kong Chian Wing, Level 2
Abstract not available.
Title: Magic Square, Quantum Mathematics, and Computing Theory
Date/Time: 08-Dec, 04:00PM
Venue: NUS Kent Ridge, University Hall Auditorium, Level 2, Lee Kong Chian Wing
Abstract not available.
Title: Sculpting a spinor condensate: from Skyrmions to quantum memory
Date/Time: 19-Nov, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: The spin and orbital angular momentum of a spinor Bose-Einstein condensate can be controlled using optical fields that carry orbital angular momentum. I will describe how this process works and our experimental progress using this approach to manipulate the BEC wavefunction. In particular, I will describe the creation of complex spin textures, coreless vortices and Skyrmions. I will also describe the application of this work to the realization of a robust memory for quantum information processing.
Title: Exploring Flatland with cold atoms
Date/Time: 29-Oct, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: In his world-famous novel "Flatland" published in 1884, the English writer Edwin Abbott imagined a social life in a two-dimensional world. With a very original use of geometrical notions, E. Abbott produced a unique satire of his own society. Long after Abbott's visionary allegory, Microscopic Physics has provided a practical path for the exploration of low-dimensional worlds. With the realization of quantum wells for example, it has been possible to produce two-dimensional gases of electrons. The properties of these fluids dramatically differ from the standard three-dimensional case, and some of them are still lacking a full understanding. During the last decade, a novel environment has been developed for the study of low-dimensional phenomena. It consists of cold atomic gases that are confined in tailor-made electromagnetic traps. With these gases, one hopes to simulate and understand more complex condensed-matter systems. The talk will discuss some aspects of this research, both from an experimental and a theoretical perspective. Connections with other domains of many-body physics, such as the Quantum Hall phenomenon, will also be addressed.
Title: Coherence and control of single electron spins in quantum dots
Date/Time: 03-Sep, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Individual electron spins isolated in semiconductor quantum dots are natural two-level quantum systems that could form the basis of a quantum information processor. Using a fully electrical approach, it is now possible to initialize, coherently manipulate and read out the spin state of a single electron in a quantum dot, and to couple it coherently to the spin of an electron in a neighbouring dot. Furthermore, we have come to a quantitative understanding of the timescales and mechanisms by which the spin loses phase coherence. Ongoing work focuses on integrating all buildings blocks in a single experiment, and on either control or elimination of the electron spin environment, in particular the nuclear spins in the quantum dot host material. This should permit using entangled spins as a new resource for quantum information processing. References R. Hanson, L.P Kouwenhoven, J.R. Petta, S. Tarucha, and L.M.K. Vandersypen, Spins in few-electron quantum dots, Reviews of Modern Physics 79, 1217 (2007) I. T.Vink, K. C. Nowack, F. H. L. Koppens, J. Danon, Yu. V. Nazarov, and L. M. K. Vandersypen, Locking electron spins into magnetic resonance by electron-nuclear feedback, arXiv:0902.2659 F.H.L. Koppens, K.C. Nowack, and L.M.K. Vandersypen, Spin echo of a single electron spin in a quantum dot, Phys. Rev. Lett. 100, 236802 (2008) K.C. Nowack, F.H.L. Koppens, Yu.V. Nazarov and L.M.K. Vandersypen, Coherent control of a single electron spin with electric fields, Science 318, 1430 (2007) F.H.L. Koppens, C. Buizert, K.J. Tielrooij, I.T. Vink, K.C. Nowack, T. Meunier, L.P. Kouwenhoven, and L.M.K. Vandersypen, Driven coherent oscillations of a single electron spin in a quantum dot, Nature 442, 766-771 (2006)
Title: AtomChips: Integrated circuits for matter waves
Date/Time: 16-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: AtomChips aim at the miniaturization and integration of quantum optics and atomic physics on to a single chip, analogous to electronic circuits. It combines the best of both worlds: The perfected manipulation techniques from atomic physics with the capability of nanofabrication. AtomChips promise to allow coherent manipulation of matter waves on the quantum level by using high spatial resolution electro magnetic potentials from structures on the atom chip or by employing adiabatic radio frequency (RF) or micro wave (MW) potentials [1]. The talk will give an overview of the recent advances in the concepts, fabrication and experimental realization of AtomChips by illustrating the many different tasks that can be performed using ultra cold or Bose-Einstein condensed (BECs) atoms manipulated on the chip. These range from measuring magnetic and electric fields with unprecedented sensitivity by observing the density modulations in trapped highly elongated 1d BECs [2], to fundamental studies of the universal properties in low dimensional systems like non equilibrium dynamics and coherence decay [3] or signatures of thermal and quantum noise [4] in one dimensional super fluids. This work was supported by the European Union integrated project SCALA, the DIP the FWF and the Wittgenstein Prize. [1] T. Schumm et al. Nature Physics, 1, 57 (2005); S. Hofferberth et al. Nature Physics, 2, 710 (2006) [2] St. Wildermuth et al. Nature 435, 440 (2005); S. Aigner et al. Science 319, 1226 (2008) [3] S. Hofferberth et al. Nature 449, 324 (2007) [4] S. Hofferberth et al. Nature Physics, 4, 489 (2008)
Title: Exploring the quantumness of light in a cavity
Date/Time: 02-Apr, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Cavity QED experiments are described, in which a beam of Rydberg atoms is used to manipulate and probe non-destructively microwave photons trapped in a very high Q superconducting cavity. We realize ideal quantum non-demolition (QND) measurements of photon numbers, observe the radiation quantum jumps due to cavity relaxation and prepare non-classical fields such as Fock and Schrödinger cat states. Combining QND photon counting with a homodyne mixing method, we reconstruct the Wigner functions of these non-classical states and, by taking snapshots of these functions at increasing times, obtain movies of the decoherence process. We also observe that the coherent evolution of the field in the cavity is frozen when we measure non-destructively its photon number and we realize in this way a simple demonstration of the quantum Zeno effect. These experiments open the way to the implementation of quantum feedback procedures aimed at preserving over long time intervals the quantum coherence of non-classical states of radiation in a cavity.
Title: Black holes as mirrors
Date/Time: 26-Mar, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: I'll discuss information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing. Instead of locking away information for a near eternity, black holes can reveal their information in Hawking radiation remarkably quickly. The resulting estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementarity hypothesis. The reason these conclusions hold is that typical local quantum circuits generate highly efficient quantum error-correcting codes. (Joint work with John Preskill)
Title: The photon and the vacuum cleaner
Date/Time: 12-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Photonic quantum technologies are largely based on the effects of quantum interference coupled with the nonlinearities that can be induced by measurements. This means that attention must be paid to the character of the individual light particles that are used to build up large-scale entanglement. In turn, this requires the development of sources and detectors that can verify the quantum state of the light field in all its degrees of freedom. I shall discuss how "vacuum engineering" can be used to prepare pure-state single photon wavepackets using nonlinear optics and conditional detection. The efficacy of such methods of preparation is predicated on the ability to understand what it is that the detector is measuring, and I shall illustrate a procedure for fully characterizing a quantum detector using tomography, thereby verifying the measurement operations for the device.
Title: Ciphering Classical Chinese
Date/Time: 05-Feb, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract: Once premodern European and Arabic cipherers started to leave the more pedestrian "physical" approaches to steganography behind, techniques of encryption in political, military, religious or private contexts consisted mainly in the transformative manipulation of a plaintext and the devision of adequate decoding keys. They were thus by and large tantamount to the evolution of ever more powerful mapping algorithms, and dependent on increasingly sophisticated insights into the distributional properties of letters and lexical roots occuring in alphabetically represented written language. In China, the topic of information secrecy has been discussed since the pre-imperial period. State institutions maintaining certain types of government information secret and laws regulating the punishment of its leakage were of great complexity already during the Qin and Han dynasties, and reached unsurpassed bureaucratic intricacies under the Ming. Nevertheless, actual coding procedures seem to have developed quite slowly throughout imperial Chinese history, in the public as well as the private sectors. After a short overview of the earliest anecdotal evidence related to the disguising of information in early Chinese military texts, my talk will concentrate on three techniques, which illustrate the difficulty of pre-modern encryption on the basis of a logographic writing system representing a largely monosyllabic, tonal language. These are (1) the so-called "character verification" method, described in the Northern Song Essentials from the Military classics by Zeng Gongliang (999-1078), which maps numbered military commands onto sequences of characters appearing in a memorized pentasyllabic poem, serving as the key; (2) a system inspired by the phonological categories of the Middle Chinese rhyme dictionary tradition, and ascribed to the famous anti-Japanese Ming general Qi Jiguang (1528-1588); (3) homophonic substitution and “synthanalytic” character manipulation in poetry and edicts, recorded, e.g. in documents from the Taiping rebellion. In a concluding discussion, the inhibitive role of the non-alphabetic writing system in the development of formalized coding procedures will be weighed against the influence, which the “non-development of probabilistic thinking“, i.e. the “lack of an exploratory serious playfulness“ (Mark Elvin) in China might have brought about.
Title: Quantum algorithm for solving linear systems of equations
Date/Time: 20-Jan, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract not available.
Title: Schrodinger's cats and kittens : new tools for quantum communications
Date/Time: 03-Dec, 04:00PM
Venue: NUS University Hall Auditorium, Lee Kong Chian Wing, Level 2
Abstract: We describe recent experiments [1, 2, 3] manipulating the quantum state of femtosecond light pulses, in order to generate photon number states (Fock states with n= 1 or 2 photons), quantum superpositions of coherent states (Schrödinger's cats and kittens [1, 2]), and non- gaussian entangled states [3]. We will also discuss possible applications of these new states for quantum communications. Wigner functions References [1] A. Ourjoumtsev et al., "Generating optical Schrödinger kittens for quantum information processing", Science 312: 83-86 (2006 ) [2] A. Ourjoumtsev et al., "Generation of optical "Schrödinger cats' from photon number states Nature 448: 784-786 (2007) [3] A. Ourjoumtsev et al. "Increasing entanglement between Gaussian states by coherent photon subtraction", PRL 98:030502 (2007)
Title: Precision Quantum Metrology
Date/Time: 02-Dec, 05:00PM
Venue: NUS University Hall Auditorium, Lee Kong Chian Wing, Level 2
Abstract: Quantum state engineering of ultracold matter and precise control of optical fields have allowed accurate measurement of light-matter interactions for applications ranging from precision tests of fundamental physics to quantum information science. State-of-the-art lasers now maintain optical phase coherence over one second. Optical frequency combs distribute this optical phase coherence across the entire visible and infrared parts of the electromagnetic spectrum, leading to direct visualization and measurement of light ripples. A new generation of light-based atomic clocks has been developed, with ultracold Sr atoms confined in a carefully engineered optical lattice offering unprecedented coherence times for light-matter interactions. The uncertainty of this new clock has reached 1 × 10-16, a factor of 4 below the current best Cs primary standard. These developments represent a remarkable convergence of ultracold science, laser technology, and ultrafast science. Further improvements are still tantalizing, with quantum measurement and precision metrology combining forces to explore the next frontier.
Title: From Einstein Intuitions to Quantum Bits: A New Quantum Age?
Date/Time: 02-Dec, 04:00PM
Venue: NUS University Hall Auditorium, Lee Kong Chian Wing, Level 2
Abstract: In 1935, with co-authors Podolsky and Rosen, Einstein discovered an amazing quantum situation, where particles in a pair are so strongly correlated that Schrödinger called them “entangled”. By analyzing that situation, Einstein concluded that the quantum formalism was incomplete. Niels Bohr immediately opposed that conclusion, and the debate lasted until the death of these two giants of physics, in the 1950’s. In 1964, John Bell produced his famous inequalities which would allow experimentalists to settle the debate, and to show that the revolutionary concept of entanglement is indeed a reality. Based on that concept, a new field of research has emerged, quantum information, where one uses entanglement between qubits to develop conceptually new methods for processing and transmitting information. Large scale practical implementation of such concepts might revolutionize our society, as did the laser, the transistor and integrated circuits, some of the most striking fruits of the first quantum revolution, which began with the 20th century.
Title: Bose-Einstein Condensation, Classical Fields and Decaying Vortices
Date/Time: 02-Oct, 04:00PM
Venue: Physics Conference Room S13, Level M
Abstract: While all experiments with ultra cold atomic gases are performed at finite, nonzero temperatures most theory papers are assuming a temperature of zero Kelvin. We have developed a simple approximation which is applicable to a gas at nonzero temperature. It treats the atomic field as a c-number rather than as an operator. We call this “the classical field approximation”. In my lecture I will explain the classical field approximation. I will compare this approximation and the exact quantum solution for a simple case of the ideal Bose gas. In the last part I will apply the method to multiply charged vortices, where some recent experiments are available. Recommended Pre-reading Materials: M. Brewczyk, M. Gajda, and K. Rzążewski, Classical fields approximation for bosons at nonzero temperature, Journ. of Physics B 40, R1-R37 (2007)
Title: Optical Lattice Immersions for Direct Quantum Simulations
Date/Time: 28-Aug, 04:00AM
Venue: Physics Conference Room S13, Level M
Abstract: I will investigate the properties of atoms which are trapped in an optical lattice and immersed into a Bose-Einstein condensate (BEC). I will show that interspecies interactions lead to dephasing of the lattice atoms and, via BEC phonons, mediate an attractive interaction between them. This may cause lattice atoms to aggregate into a cluster. I will also study the impact of the BEC on transport properties and find a crossover from coherent behaviour described by an extended Hubbard model to diffusive hopping. Furthermore I will show that weak attractive interspecies interactions may give rise to instabilities. I will discuss the relation of this atomic lattice immersion to condensed matter systems and how it may be used for direct quantum simulations. Finally, I will extend these considerations to moving and rotating Bose-Einstein condensates and show how magnetic phenomena can be simulated in lattice immersions.
Title: Quantum Phase Transitions in Arrays of Coupled QED Cavities
Date/Time: 17-Jul, 04:00PM
Venue: Physics Conference Room S13, Level M
Abstract: Recent proposals of realizing condensed phases in cavity-QED like systems opened a number of new exciting possibilities in the physics of strongly interacting photonic systems. Arrays of coupled QED cavities have been shown to have superfluid, insulating and glassy phases. They can be used for transferring quantum information and for simulating interacting spin systems. Coupled cavities can be realized in a wide range of physical systems, from nanocavities in photonic crystals to Cooper pair boxes in superconducting resonators. After a brief introduction to the field, I will review the main features of the phase diagram and of the signatures of the various phases. I will discuss how, in principle, it might be possible to distinguish between the different phases by measuring photons fluctuations. I will finally discuss dynamical instabilities in these arrays when control parameters are varied in time.
Title: Open Quantum Systems, Entanglement, and the Laser Quantum State
Date/Time: 29-May, 04:00PM
Venue: Physics Conference Room S13, Level M
Abstract: An introduction to the treatment of open quantum systems as stochastic scattering processes (quantum trajectories) will be presented, together with some applications to illustrate the ideas. The final application will address the question: is optical coherence in fact a fiction [Moelmer, PRA 55, 3195 (1997)] or rather more fact than fiction [Noh and Carmichael, PRL 100, 120405 (2008)]?
Title: Cavity QED: Quantum Control with Single Atoms and Single Photons
Date/Time: 17-Apr, 04:00PM
Venue: S16 - LT31
Abstract: Over the past two decades, strong interactions of light and matter at the single-atom and single-photon level have enabled a wide set of scientific advances in quantum optics and quantum information science. Single, ultra-cold atoms can now be made to interact strongly and controllably with single-photon light fields confined within microscopic optical resonators (cavity quantum electrodynamics, or cavity QED). Moreover, new resonator configurations, such as lithographically-fabricated monolithic microresonators, hold great promise for the implementation of quantum networks and quantum logic with atoms and photons. In this colloquium I describe some elementary cavity QED systems and potential applications of these systems in quantum information processing. This includes recent theoretical and experimental results for cavity QED with toroidal microresonators.
Title: New Developments in Measurement-Based Quantum Computation
Date/Time: 27-Mar, 04:00PM
Venue: S16 - LT31
Abstract: Quantum computers offer a promising new way of information processing, in which the distinguishing features of quantum mechanics can fruitfully be exploited. Next to the standard quantum circuit model, various other models for quantum computation exist. Although these models have been shown to be formally equivalent, their underlying elementary concepts, as well the requirements for their practical realization, differ significantly. Exciting perspectives are offered by the new paradigm of measurement-based quantum computation, where the processing of quantum information takes place by rounds of simple measurements on a system of spins prepared in a highly entangled state. In this talk I will discuss a number of recent developments in measurement-based quantum computation on both fundamental and practical issues, e.g. regarding the power of quantum computation and its relation to entanglement, as well as steps toward its experimental realization. Furthermore, I will highlight the various ways in which this field is connected to other branches in physics and mathematics.
Title: Implementation of Basic Quantum Information Processing
Date/Time: 28-Feb, 04:00PM
Venue: S16 - LT31
Abstract: Quantum information processing is one of the most promising research areas for its possibility of bringing a revolutionary change in the information society. To realize it, however, there are several fundamental issues to be examined, namely, how to protect the quantum information from environmental noise, how to realize one-way computation using the cluster states, and how to guarantee the security of quantum cryptography under realistic noise and imperfection of the devices. In this talk I will introduce the basics of these issues and will describe the possible solutions mainly with photons including activity in my research group in Osaka University.
Title: Quantum Teleportation and Nonlocality
Date/Time: 10-Jan, 04:00PM
Venue: CQT Seminar Room, S15-03-15
Abstract not available.