February 18, 2016:
Random words, longest increasing subsequences, and quantum PCA
by
Ryan O'Donnell, Carnegie Mellon University


March 10, 2016:
Leibniz on Complexity
by
Gregory Chaitin, Federal University of Rio de Janeiro


March 30, 2016:
Photonic Crystals and Photonic Molecules at Telcom Wavelengths
by
Robert Taylor, University of Oxford


April 7, 2016:
Shedding Starlight on the Quantum Fabric of Spacetime
by
Giovanni AmelinoCamelia, University of Rome La Sapienza


April 14, 2016:
Fundamental tests of nature with cooled and stored exotic ions
by
Klaus Blaum, Max Planck Institute for Nuclear Physics


July 28, 2016:
Quantum coherence in photosynthetic proteins: insights for emerging energy technologies
by
Alexandra OlayaCastro, University College London


September 22, 2016:
Antihydrogen  a tool to study matterantimatter symmetry in the laboratory
by
Eberhard Widmann, Austrian Academy of Sciences


November 10, 2016:
Magnetic imaging with point defects in diamond
by
JeanFrançois ROCH, Université ParisSaclay


November 17, 2016:
Exploring quantum matter with photons
by
Jake Taylor, Joint Center for Quantum Information and Computer Science, Joint Quantum Institute, National Institute of Standards and Technology


Dec 7, 2016:
CQT Annual Symposium: The Famous, The Bit & The Quantum
The Quantum Approximate Optimization Algorithm: A Good Choice to Run on a Near Term Quantum Computer
by Edward Farhi, MIT, USA
From Quantum Philosophy to Quantum Technology
by Markus Arndt, University of Vienna, Austria


Date: 18 February 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: Ryan O'Donnell, Carnegie Mellon University
Random words, longest increasing subsequences, and quantum PCA
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 lowrank, 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.
Date: 10 March 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: Gregory Chaitin, Federal University of Rio de Janeiro
Leibniz on Complexity
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.
Date: 30 March 2016 (Wednesday), 12pm
Venue: CQT Seminar Room, S150315
Speaker: Robert Taylor, University of Oxford
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).
Date: 7 April 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: Giovanni AmelinoCamelia, University of Rome La Sapienza
Shedding Starlight on the Quantum Fabric of Spacetime
Abstract:
Over the last decade there has been an intense effort using observations in astrophysics for setting constraints, with Plancklength sensitivity, on the properties of the laws of propagation of particles in a quantum spacetime. Importantly this has led to the demise of an oldfashioned naïve assessment according to which Planckscale 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" (nonsystematic quantumspacetime effects) significant improvements are expected for searches of the associated decoherence effects.
Date: 14 April 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: Klaus Blaum, Max Planck Institute for Nuclear Physics
Fundamental tests of nature with cooled and stored exotic ions
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 highaccuracy mass measurements of shortlived radionuclides, gfactor determinations of the boundelectron in highlycharged, hydrogenlike ions and gfactor 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 gfactor measurements.
Date: 28 July 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: Alexandra OlayaCastro, University College London
Quantum coherence in photosynthetic proteins: insights for emerging energy technologies
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 quasicoherently 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 bioinspired energy technologies.
Date: 22 September 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: Eberhard Widmann, Austrian Academy of Sciences
Antihydrogen  a tool to study matterantimatter symmetry in the laboratory
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 matterantimatter 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 electromagnetic 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 groundstate 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 ultracold 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.
Date: 10 November 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: JeanFrançois ROCH, Université ParisSaclay
Magnetic imaging with point defects in diamond
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 nitrogenvacancy (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 plasmaassisted diamond growth and nanoscale ion implantation.
Date: 17 November 2016, 4pm
Venue: CQT Seminar Room, S150315
Speaker: Jake Taylor, Joint Center for Quantum Information and Computer Science, Joint Quantum Institute, National Institute of Standards and Technology
Exploring quantum matter with photons
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 photonphoton interactions, merging the best coherence properties of light with manybody physics. I will discuss our efforts for realizing nontrivial phases of matter using optical and microwavedomain photons. While our ability to control the singleparticle properties of light via photonic crystal and metamaterial techniques provides a useful tool kit for singleparticle physics, going to the manybody 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.