Description

This is a new cross-disciplinary sub-project, running in parallel with Quantum Simulations with Strongly Interacting Photons to support the ongoing collaboration by Dimitris (theory) and Bjorn (micro-traps experiment). The main idea is to establish a collaboration in the field of many body cavity QED effects. Our main objective would be to develop the theoretical tools to characterize the plethora of many body physics possible to realize in the Micro-traps experiments, especially related to the recent accomplishement of a BEC creation in a micro-cavity. We will start by exploring the possibility to develop non-destructive optical methods to characterize the state of the atomic enseble. This collaboration will also provide the chance to test in an existing experimental platform some of the results obtained in the parallel theory project of Quantum Simulations with Strongly Interacting Photons.

CQT people involved in the project

Dimitris G. Angelakis, Bjorn Hessmo, Kwek Leong Chuan, Murray Barrett

Publications

Description

Physical systems in which quantum simulators have successfully been demonstrated include so far Josephson arrays, ions traps and optical lattices (see the reviews R. Fazio and H. van der Zant, Phys. Rep. 355, 235 (2001) and M. Lewenstein et al., Adv. Phys. 56, 243 (2007)). Can there be another “engineered” quantum many-body system which could be used as an efficient quantum simulator? This will be especially interesting if the strengths of this system are ``complementary" to that of optical lattices -- for example if it allowed the co-existence of accessibility to individual constituents of a many-body system and a strong interaction between them, or if it allowed the simulation of arbitrary networks rather than those derivable from superposing lattices. Particularly arresting will be to find such phases by modifying a system of photons which, by being non-interacting, are naively expected to be unlikely candidates for the studies of many-body phenomena. These questions lie at the crux of this proposal.

CQT people involved in the project

Dimitris G. Angelakis, Kwek Leong Chuan, Elica Kyoseva, Dai Li, Dagomir Kaszlikowski, Vlatko Vedral

Publications

• Coherent control of steady state entanglement in driven cavity arrays
  Dimitris G. Angelakis, Li Dai, LC Kwek,
  arXiv:0906.2168.
• Steady state entanglement between distant hybrid light-matter qubits under classical driving.
  Dimitris G. Angelakis, Stefano Mancini, Sougato Bose,
  Eur. Phys. Lett. 85 20007 (2009). (arXiv:0711.1830)).
• Fractional Quantum Hall state in coupled cavities.
  Jaeyoon Cho, Dimitris G. Angelakis, Sougato Bose,
  Phys. Rev. Lett. 101, 246809 (2008) [http://arxiv.org/abs/0807.1802 arXiv:0807.1802]
• Heralded generation of two-photon polarization entanglement with coupled cavities.
  Jaeyoon Cho, Dimitris G. Angelakis, Sougato Bose,
  Phys. Rev. A 78 022323 (2008) (arXiv:0712.2413).
• Reproducing spin lattice models in strongly coupled atom-cavity systems.
  Alastair Kay and Dimitris G. Angelakis
  Eur. Phys. Lett. Eur. Phys. Lett. 84 (2008) 20001(arXiv:0802.0488).
• Simulation of high-spin Heisenberg chains in coupled cavities.
  Jaeyoon Cho, Dimitris G. Angelakis, Sougato Bose,
  Phys. Rev. A 78 062338 (2008) (arXiv:0802.3365).
• Weaving light-matter qubits into a one way quantum computer
  Dimitris G. Angelakis, Alastair Kay,
  New J. Phys. Vol. 10, 023012 (2008). (arXiv:quant-ph/0702133).
• Photon blockade induced Mott transitions and XY spin models in coupled cavity arrays.
  Dimitris G. Angelakis, Marcelo Santos Sougato Bose,
  Phys. Rev. A (Rap. Com.) vol. 76, 031805 (2007) (arXiv:quant-ph/0606159)
• A proposal for the implementation of quantum gates with photonic-crystal coupled cavity waveguides.
  Dimitris G. Angelakis, M. Santos, V. Yanopappas, A.K. Ekert,,
  Phys. Lett. A. Vol.362, 377 (2007) (arXiv:quant-ph/0410189)

Description

Unitary and spherical designs are mathematical structures efficiently approximating the notion of uniform distribution on the unitary group and quantum state space. As such they have many important applciations, as well as an intriguing mathematical theory. The intention of this project is to develop the understanding and range of applications of designs in quantum information theory; a new direction is the possible relation to random-matrix theory.

CQT people involved in the project

Andreas Winter, Markus Grassl

Publications

• A. Ambainis, J. Bouda, A. Winter. Tamper-resistant encryption of quantum information. arXiv:0808.0353.
• W. Matthews, S. Wehner, A. Winter, Distinguishability of quantum states under restricted families of measurements.

Description

Entangled quantum states give rise to correlations between distant particles, that cannot be explained either by signaling, or by pre-established strategies. In short, one says that quantum correlations are non-local. This project is centered on the study of a new approach to a quantitative study on non-locality and on the development of black-box quantum information.

CQT people involved in the project

Valerio Scarani, Timothy Liew

Publications

• How non-local are n noisy Popescu-Rohrlich Machines?
  M. Fitzi, E. Hanggi, V. Scarani, S. Wolf
  arXiv:0811.1649
• Simulation of partial entanglement with nonsignaling resources
  Nicolas Brunner, Nicolas Gisin, Sandu Popescu, Valerio Scarani
  Phys. Rev. A 78, 052111 (2008) arXiv:0803.2359
• Testing the Hilbert space dimension
  Nicolas Brunner, Stefano Pironio, Antonio Acin, Nicolas Gisin, Andre Allan Methot, Valerio Scarani
  Phys. Rev. Lett. 100, 210503 (2008) arXiv:0802.0760
• Local and nonlocal content of bipartite qubit and qutrit correlations
  Valerio Scarani
  Phys. Rev. A 77, 042112 (2008) arXiv:0712.2307

Description

Any realization of quantum cryptography produces a key of finite length. Theoretical security bounds, however, have been derived only for infinitely long keys. This project aims at deriving security bounds for finite keys, thus closing one of the open gaps between theory and practical realizations.

CQT people involved in the project

Valerio Scarani, Raymond Cai

Publications

• Finite-key analysis for practical implementations of quantum key distribution
  R.Y.Q. Cai, V. Scarani
  arXiv:0811.2628
• Security Bounds for Quantum Cryptography with Finite Resources
  V. Scarani, R. Renner
  Proceedings of TQC 2008, Lecture Notes in Computer Science 5106 (Springer Verlag, Berlin), pp.83-95 (2008) arXiv:0806.0120
• Experimental quantum key distribution based on a Bell test.
  Alexander Ling, Matthew P. Peloso, Valerio Scarani, Antia Lamas-Linares and Christian Kurtsiefer.
  Phys. Rev. A 78, 020301(R) (2008) arXiv:0805.3629
• Quantum cryptography with finite resources: Unconditional security bound for discrete-variable protocols with one-way post-processing
  V. Scarani, R. Renner
  Phys. Rev. Lett. 100, 200501 (2008) arXiv:0708.0709
• A framework for practical quantum cryptography
  Valerio Scarani, Helle Bechmann-Pasquinucci, Nicolas J. Cerf, Miloslav Dusek, Norbert Lutkenhaus, Momtchil Peev
  Accepted in Rev. Mod. Phys. arXiv:0802.4155

Description

Entanglement is a fundamental resource in quantum information and computation science. In this project we are interested in how entanglement is related to thermodynamic behaviour of many-body quantum systems. In particular, we are investigating a connection between entanglement and different phases of matter as well as in possible ways to utilize entanglement present in macroscopic systems.

CQT people involved in the project

D. Kaszlikowski, V. Vedral, A. Winter, M. Wiesniak, R. Ramanathan

Publications

• D. Kaszlikowski, M. Wiesniak, "Off-Diagonal-Long-Range-Order Versus Entanglement", arXiv:0712.0990 (2007).
• D. Kaszlikowski, A. Kay, "A Witness of Multipartite Entanglement Strata", arXiv:0710.1928 (2007).
• D. Kaszlikowski, A. Sen (De), U. Sen, V. Vedral, A. Winter, "Quantum Correlation Without Classical Correlations?", arXiv:0705.1969 (2007).
• D. Kaszlikowski, W. Son, V. Vedral, "Dimensionality induced entanglement in macroscopic dimer systems", Phys. Rev. A 76, 054302 (2007).

Description

This project is primarily intended to explore single copy deterministic transformations between multipartite entangled pure states. It is well understood whether, and how, two bipartite states can be interconverted either deterministically or probabilistically, and this forms the basis for our understanding of bipartite entanglement.

CQT people involved in the project

D. Kaszlikowski, Kwek Leong Chuan

Publications
We are working on it.

Description

Entanglement has been found to exist in many macroscopic physical systems at finite temperature. However, most, if not all, of these studies are performed in thermodynamic equilibrium. Natural systems, on the other hand, are hardly ever in equilibrium. This project aims at exploring the behaviour of entanglement and other information theoretic measures in non-equilibrium thermodynamics.

CQT people involved in the project

Vlatko Vedral, Libby Heaney, Kavan Modi, Wonmin Son, Elisabeth Rieper

Publications

• V. Vedral, Entanglement production in non-equilibrium thermodyanmics, arXiv:0706:3183v1
• K. Maruyama, F. Nori and V. Vedral, Physics of Maxwell's demon and information, arXiv:0707:3400v1, to appear in Rev. Mod. Phys.
• W. Son, D. Kaszlikowski and V. Vedral, Phys. Rev. A. 76, 054302 (2007).
• J. Dunningham and V. Vedral, Phys. Rev. Lett. 99, 180404 (2007).
• J. Hide, W. Son and V. Vedral, Enhancing Natural Entanglement with Disorder, submitted to Physical Review Letters(2008).
• G. Vacanti, M. Paternostro, M. Palma and V. Vedral, Opto-mechanical to mechanical entanglement transformation, to appear in New. J. Phys. (2008).
• W. Son, L.Amico, F. Plastina and V. Vedral, Quantum instability in a quasi-long-range ordered phase, submitted to Phys. Rev. Lett. (2008).

Cluster state quantum computation is an alternative, though equivalent in terms of power, model to the standard gate model of quantum computation. Its advantage is that entanglement is invested at the very beginning of the computation and all subsequent steps are done by using one qubit measurements only. This project investigates the performance of such computers under noise and within various different scenarios.

CQT people involved in the project

Vlatko Vedral, Wonmin Son, Libby Heaney

Publications

• J. Anders, M. Hajdusek, D. Markham and V. Vedral, How much of one-way computation is just thermodynamics, arXiv:0702020 [[1]] , to appear in Foundations of Physics (2008).
• V. Vedral, Quantifying Entanglement in Macroscopic Systems, NATURE 453, 1004-1007(2008).
• D. Markham, J. Anders, V. Vedral, M. Murao and A. Miyake, Survival of entanglement in thermal states, Eu. Phys. Lett. 81, 40006, (2008).
• W. Son, J. Kofler, M. S. Kim, V. Vedral and C. Brukner, Positive Phase Space Transformation Incompatible with Classical Physics, PHYSICAL REVIEW LETTERS 102, 110404 (2009).

Description

The uncertainty principle is one of the cornerstones of quantum mechanics. Since Heisenberg's 1927 paper numerous mathematical formulations as uncertainty relations have been found; the entropic versions being the truly information theoretic ones. These relations, the strongest known due to Maassen and Uffink, also have striking quantum information and cryptography applications, and in this project we will aim to extend these results beyond the case of two observables, which has barely been attempted until now.

CQT people involved in the project

Andreas Winter

Publications
S. Wehner and A. Winter, Higher entropic uncertainty relations for anti-commuting observables, arXiv:0710:1185 [[1]]

Description

The so-called additivity-conjectures of quantum information theory concern the mathematical proprty of extensivity of a number of entropic parameters associated to channels and (bipartite) quantum states: the minimum output entropy, the Holevo-quantity and the entanglement of formation, among others. All of them are long conjectured to be additive (extensive) for tensor products of channels/states -- in fact, Shor showed in 2002 that all their additivities are equivalent. However, recently new doubts about these conjectures arose when examples were found where the minimum p-Renyi output entropy of channels is not additive, for p>1, and then even some p<1. This project will investigate these counterexamples further, find new ones and attempt to make progress with the original conjecture (limiting case of p=1).

CQT people involved in the project

Andreas Winter

Publications

• A. Winter, The maximum output p-norm of quantum channels is not multiplicative for any p>2, arXiv:0707.0402 [[1]]
• T. Cubitt, A. W. Harrow, D. Leung, A. Montanaro and A. Winter, Counterexamples to additivity of minimum output p-Renyi entropy for p close to 0, arXiv:0710.1185 [[2]]

Description

Systems of many interacting particles are of evident interest in physics, for instance as microscopic description of macroscopic objects, say in statistical mechanics. Quantum physics is no exception, and in addition to the classical correlators studied in quantum many-body systems, recently also the entanglement properties of such systems are studied. Here we want, in parallel to the corresponding theory of genuine n-party entanglement, investigate reasonable and axiomatic ways to define genuine n-party classical correlations. Our publication 1. points to exciting conceptual applications of such an axiomatization.

CQT people involved in the project

Andreas Winter, Dagomir Kaszlikowski

Publications
D. Kaszlikowski, A. Sen(De), U. Sen, V. Vedral and A. Winter, Quantum Correlations without Classical Correlations?, arXiv:0705.1969 [[1]]

Description

The concept of entanglement is a negative: the non-factorizable states are called entangled. Furthermore, measures of entanglement are required to be monotonic under classes of operations including LOCC (=local operations and classical communication). Hence, the properties of entanglement are really just the reflection of the structure of the class LOCC of cptp maps. In this project we want to understand this class better, along various lines of asking about the power of LOCC to perform certain information theoretic or statistical tasks.

CQT people involved in the project

Andreas Winter

Publications

• W. Matthews and A. Winter, On the Chernoff distance for asymptotic LOCC discrimination of bipartite quantum states, arXiv:0710.4113 [[1]]
• W. Matthews and A. Winter, PPT pure state transformations and catalysis, arXiv:0801.4322 [[2]]

Description

The primary objective of our studies is to achieve a deeper theoretical understanding of the geometric phase and its implementation as fast and robust quantum gates in quantum circuits. Several approaches will be explored, including but not limited by the following:

When we have two or more systems interacting - such as an atom and a quantized field, we can construct a whole range of different geometric phases which can be used to simulate anyonic phases. We shall also investigate the geometric phases induced by quantized fields, simulate two-anyon statistics and study collective effects of many anyons: entanglement and phase transitions

The issue of geometric phase for open system is important since all physical systems are essentially open. Although there have been some attempts to address this issue, a complete understanding is somewhat lacking. A good understanding will indeed provide impetus for new ideas in fault tolerant quantum computations.

It is envisaged that quantum operations can take advantage of non-Abelian (Yang-Mills) gauge potentials which may provide extra features and may suppress decoherence. We propose to study how a desired Yang-Mills gauge potential may be realized by coupling a fast quantum system to a slow system (which could be described also classically).



CQT people involved in the project

Oh Choo Hiap, Vlatko Vedral, Lai Choy Heng, Kuldip Singh, Kwek Leong Chuan

Description

Entanglement is generated 'naturally' when physical systems interact coherently. For example two electrons trapped at nearby sites can entangle with one another through their mutual Heisenberg interaction. This is the basis of many schemes for QIP, especially in the solid state. However, there is another route to entanglement: rather than allowing the physical qubits to interact directly, instead they can be kept at fixed, well separated sites and simply subjected to a measurement. The resulting paradigm of distributed QIP is increasingly perceived as an excellent prospect for a real practical technology. In this theoretical project we will examine a range of issues related to the distributed approach, from the physics of the individual subsystems through to the properties of the large scale entangled states which one would aim to create.



CQT people involved in the project

Kwek Leong Chuan, Simon C. Benjamin, Dagomir Kaszlikowski.

Collaborators

Dan Browne (UCL), Brendon Lovett (Oxford), Sougato Bose (UCL), Marshall Stoneham (UCL); Dimitris Angelakis (Univ. Chania, Crete).

Publications

• Avinash Kolli, Simon C. Benjamin, Brendon W. Lovett, Thomas M. Stace, Measurement-based approach to entanglement generation in coupled quantum dots, arxiv.org:0801.2873 [[1]] .
• Earl T. Campbell, Simon C. Benjamin, Measurement based entanglement under conditions of extreme photon loss, arxiv.org:0710.4352 [[2]] .
• Avinash Kolli, Simon C. Benjamin, Jose Garcia Coello, Sougato Bose, Brendon W. Lovett, Large Spin Entangled Current from a Passive Device, arxiv.org:0801.4411 [[3]] .

Description

We address various questions concerning the retrieval of quantum information stored in physical systems, as well as the physical realization of robust systems for storing and communicating quantum information.

CQT people involved in the project

Berge Englert, Philippe Raynal, Amir Kalev, Andreas Keil, Syed Assad, Zhu Huangjun, Han Rui, Gelo Macuja

Publications

• Symmetric construction of reference-frame-free qudits
  J. Suzuki, G. N. M. Tabia, B.-G. Englert
  arXiv:0802.1609[quant-ph].
• Wave-particle duality in multi-path interferometers: General concepts and three-path interferometers
  B.-G. Englert, D. Kaszlikowski, L. C. Kwek, and W. H. Chee, International Journal of Quantum Information 6, 129-157 (2008).
• Accessible information about quantum states: An open optimization problem
  J. Suzuki, S. M. Assad, and B.-G. Englert
• Chapter 11 in Mathematics of Quantum Computation and Quantum Technology, edited by G. Chen, S.J. Lomonaco, and L. Kauffman (Chapman & Hall/CRC, Boca Raton 2007), pp. 309-348.
• Iterative procedure for computing accessible information in quantum communication
  J. Rehacek, B.-G. Englert, and D. Kaszlikowski
  Physical Review A 71, 054303 (2005).

Description

In the circuit model of Quantum Computation (QC), all qubits are initially prepared in a default state, are then manipulated by a sequences of unitary single-qubit and two-qubit gates, and are finally measured in the computational basis. By contrast, in measurement-based QC one first prepares an enormous many-qubit entangled state, and then carries out a sequence of single-qubit measurements, eventually followed, if necessary, by single-qubit rotations as the final step. Since some multi-qubit gates are more easily implemented in the circuit model, and others in measurement-based QC, we investigate the potential advantages of Hybrid QC, which combines the two approaches in an attempt to make the best use of the particular advantages that each offers.

CQT people involved in the project

Berge Englert, Arun, Le Huy Nguyen

Publications
None as yet

Description

This joint project with CNRS comprises theoretical and experimental studies of ultracold degenerate Fermi gases in two dimensions.

CQT people involved in the project

Berge Englert, Christian Miniatura, Benoît Grémaud, Lee Kean Loon, Marta Wolak, Wang Guangquan, Han Rui, Bess Fang

Publications

• Pair formation and collapse in imbalanced Fermion populations with unequal masses
  G. G. Batrouni, M. J. Wolak, F. Hebert, V. G. Rousseau
  arXiv:0809.4549
• Fermionization of a strongly interacting Bose-Fermi mixture in a one-dimensional harmonic trap
  Bess Yiyuan Fang, Patrizia Vignolo, Christian Miniatura, Anna Minguzzi
  arXiv:0809.4419
• Quantum Diffusion of Matter Waves in 2D Speckle Potentials
  C. Miniatura, R.C. Kuhn, D. Delande, C.A. Mueller
  arXiv:0807.3698