Degradable quantum channels are an important class of completely positive trace-preserving maps. Among other properties, they offer a single-letter formula for the quantum and the private classical capacity and are characterized by the fact that a complementary channel can be obtained from the channel by applying a degrading channel. In this paper, we introduce the concept of approximate degradable channels, which satisfy this condition up to some finite epsilon {\textgreater}= 0. That is, there exists a degrading channel which upon composition with the channel is e-close in the diamond norm to the complementary channel. We show that for any fixed channel the smallest such e can be efficiently determined via a semidefinite program. Moreover, these approximate degradable channels also approximately inherit all other properties of degradable channels. As an application, we derive improved upper bounds to the quantum and private classical capacity for certain channels of interest in quantum communication.

%B IEEE TRANSACTIONS ON INFORMATION THEORY %V 63 %P 7832–7844 %8 dec %R 10.1109/TIT.2017.2754268 %0 Journal Article %J Physical Review X %D 2017 %T Autonomous Quantum Clocks: Does Thermodynamics Limit Our Ability to Measure Time? %A Erker, Paul %A Mitchison, Mark T. %A Silva, Ralph %A Woods, Mischa P. %A Brunner, Nicolas %A Huber, Marcus %XTime remains one of the least well-understood concepts in physics, most notably in quantum mechanics. A central goal is to find the fundamental limits of measuring time. One of the main obstacles is the fact that time is not an observable and thus has to be measured indirectly. Here, we explore these questions by introducing a model of time measurements that is complete and autonomous. Specifically, our autonomous quantum clock consists of a system out of thermal equilibrium—a prerequisite for any system to function as a clock—powered by minimal resources, namely, two thermal baths at different temperatures. Through a detailed analysis of this specific clock model, we find that the laws of thermodynamics dictate a trade-off between the amount of dissipated heat and the clock’s performance in terms of its accuracy and resolution. Our results furthermore imply that a fundamental entropy production is associated with the operation of any autonomous quantum clock, assuming that quantum machines cannot achieve perfect efficiency at finite power. More generally, autonomous clocks provide a natural framework for the exploration of fundamental questions about time in quantum theory and beyond.

%B Physical Review X %V 7 %P 031022 %U https://link.aps.org/doi/10.1103/PhysRevX.7.031022 %R 10.1103/PhysRevX.7.031022 %0 Journal Article %J Quantum %D 2017 %T Decoupling with random diagonal unitaries %A Nakata, Yoshifumi %A Hirche, Christoph %A Morgan, Ciara %A Winter, Andreas %X Yoshifumi Nakata, Christoph Hirche, Ciara Morgan, and Andreas Winter, Quantum 1, 18 (2017). https://doi.org/10.22331/q-2017-07-21-18 We investigate decoupling, one of the most important primitives in quantum Shannon theory, by replacing the uniformly distributed random unitaries commonly used to achieve the protocol, with… %B Quantum %V 1 %P 18 %8 jul %U https://quantum-journal.org/papers/q-2017-07-21-18/ %R 10.22331/q-2017-07-21-18 %0 Journal Article %J PHYSICAL REVIEW X %D 2017 %T Efficient Quantum Pseudorandomness with Nearly Time-Independent Hamiltonian Dynamics %A Nakata, Yoshifumi %A Hirche, Christoph %A Koashi, Masato %A Winter, Andreas %XQuantum randomness is an essential key to understanding the dynamics of complex many-body systems and also a powerful tool for quantum engineering. However, exact realizations of quantum randomness take an extremely long time and are infeasible in many-body systems, leading to the notion of quantum pseudorandomness, also known as unitary designs. Here, to explore microscopic dynamics of generating quantum pseudorandomness in many-body systems, we provide new efficient constructions of unitary designs and propose a design Hamiltonian, a random Hamiltonian of which dynamics always forms a unitary design after a threshold time. The new constructions are based on the alternate applications of random potentials in the generalized position and momentum spaces, and we provide explicit quantum circuits generating quantum pseudorandomness significantly more efficient than previous ones. We then provide a design Hamiltonian in disordered systems with periodically changing spin-glass-type interactions. The design Hamiltonian generates quantum pseudorandomness in a constant time even in the system composed of a large number of spins. We also point out the close relationship between the design Hamiltonian and quantum chaos.

%B PHYSICAL REVIEW X %V 7 %8 apr %R 10.1103/PhysRevX.7.021006 %0 Journal Article %J PHYSICAL REVIEW X %D 2017 %T Equilibration Time Scales of Physically Relevant Observables %A Garcia-Pintos, Luis Pedro %A Linden, Noah %A Malabarba, Artur S. L. %A Short, Anthony J. %A Winter, Andreas %XWe address the problem of understanding, from first principles, the conditions under which a quantum system equilibrates rapidly with respect to a concrete observable. On the one hand, previously known general upper bounds on the time scales of equilibration were unrealistically long, with times scaling linearly with the dimension of the Hilbert space. These bounds proved to be tight since particular constructions of observables scaling in this way were found. On the other hand, the computed equilibration time scales for certain classes of typical measurements, or under the evolution of typical Hamiltonians, are unrealistically short. However, most physically relevant situations fall outside these two classes. In this paper, we provide a new upper bound on the equilibration time scales which, under some physically reasonable conditions, give much more realistic results than previously known. In particular, we apply this result to the paradigmatic case of a system interacting with a thermal bath, where we obtain an upper bound for the equilibration time scale independent of the size of the bath. In this way, we find general conditions that single out observables with realistic equilibration times within a physically relevant setup.

%B PHYSICAL REVIEW X %V 7 %R 10.1103/PhysRevX.7.031027 %0 Journal Article %J Physical Review A %D 2017 %T Erratum: Resource theory of coherence: Beyond states [Phys. Rev. ASocial media platforms make tremendous amounts of data available. Often times, the same information is behind multiple different available data sets. This lends growing importance to latent variable models that try to learn the hidden information from the available imperfect versions. For example, social media platforms can contain an abundance of pictures of the same person, yet all of which are taken from different perspectives. In a simplified scenario, one may consider pictures taken from the same perspective, which are distorted by noise. This latter application allows for a rigorous mathematical treatment, which is the content of this contribution. We apply a recently developed method of dependent component analysis to image denoising when multiple distorted copies of one and the same image are available, each being corrupted by a different and unknown noise process. In a simplified scenario, one may assume such a distorted image to be corrupted by noise that acts independently on each pixel. We answer completely the question of how to perform optimal denoising, when at least three distorted copies are available: First we define optimality of an algorithm in the presented scenario, and then we describe an aymptotically optimal universal discrete denoising algorithm (UDDA).

%B 2017 {INTERNATIONAL} {SYMPOSIUM} {ON} {WIRELESS} {COMMUNICATION} {SYSTEMS} ({ISWCS}) %S International {Symposium} on {Wireless} {Communication} {Systems} %P 205–210 %@ 978-1-5386-2913-0 %0 Journal Article %J PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES %D 2017 %T Interferometric visibility and coherence %A Biswas, Tanmoy %A Garcia Diaz, Maria %A Winter, Andreas %X Recently, the basic concept of quantum coherence (or superposition) has gained a lot of renewed attention, after Baumgratz et al. (Phys. Rev. Lett. 113, 140401. (doi: 10.1103/PhysRevLett.113.140401)), following Aberg (http://arxiv.org/abs/quant-ph/0612146), have proposed a resource theoretic approach to quantify it. This has resulted in a large number of papers and preprints exploring various coherence monotones, and debating possible forms for the resource theory. Here, we take the view that the operational foundation of coherence in a state, be it quantum or otherwise wave mechanical, lies in the observation of interference effects. Our approach here is to consider an idealized multi-path interferometer, with a suitable detector, in such a way that the visibility of the interference pattern provides a quantitative expression of the amount of coherence in a given probe state. We present a general framework of deriving coherence measures from visibility, and demonstrate it by analysing several concrete visibility parameters, recovering some known coherence measures and obtaining some new ones. %B PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES %V 473 %R 10.1098/rspa.2017.0170 %0 Journal Article %J PHYSICAL REVIEW A %D 2017 %T Logarithmic coherence: Operational interpretation of l(1)-norm coherence %A Rana, Swapan %A Parashar, Preeti %A Winter, Andreas %A Lewenstein, Maciej %XWe show that the distillable coherence-which is equal to the relative entropy of coherence-is, up to a constant factor, always bounded by the l(1)-norm measure of coherence (defined as the sum of absolute values of off diagonals). Thus the latter plays a similar role as logarithmic negativity plays in entanglement theory and this is the best operational interpretation from a resource-theoretic viewpoint. Consequently the two measures are intimately connected to another operational measure, the robustness of coherence. We find also relationships between these measures, which are tight for general states, and the tightest possible for pure and qubit states. For a given robustness, we construct a state having minimum distillable coherence.

%B PHYSICAL REVIEW A %V 96 %R 10.1103/PhysRevA.96.052336 %0 Journal Article %J Physical Review A %D 2017 %T Non-{Gaussian} operations on bosonic modes of light: {Photon}-added {Gaussian} channels %A Sabapathy, Krishna Kumar %A Winter, Andreas %X We present a framework for studying bosonic non-Gaussian channels of continuous-variable systems. Our emphasis is on a class of channels that we call photon-added Gaussian channels, which are experimentally viable with current quantum-optical technologies. A strong motivation for considering these channels is the fact that it is compulsory to go beyond the Gaussian domain for numerous tasks in continuous-variable quantum information processing such as entanglement distillation from Gaussian states and universal quantum computation. The single-mode photon-added channels we consider are obtained by using two-mode beam splitters and squeezing operators with photon addition applied to the ancilla ports giving rise to families of non-Gaussian channels. For each such channel, we derive its operator-sum representation, indispensable in the present context. We observe that these channels are Fock preserving (coherence nongenerating). We then report two examples of activation using our scheme of photon addition, that of quantum-optical nonclassicality at outputs of channels that would otherwise output only classical states and of both the quantum and private communication capacities, hinting at far-reaching applications for quantum-optical communication. Further, we see that noisy Gaussian channels can be expressed as a convex mixture of these non-Gaussian channels. We also present other physical and information-theoretic properties of these channels. %B Physical Review A %V 95 %P 062309 %U https://link.aps.org/doi/10.1103/PhysRevA.95.062309 %R 10.1103/PhysRevA.95.062309 %0 Journal Article %J PHYSICAL REVIEW A %D 2017 %T Resource theory of coherence: Beyon} states %A Ben Dana, Khaled %A Garcia Diaz, Maria %A Mejatty, Mohamed %A Winter, Andreas %XWe generalize the recently proposed resource theory of coherence (or superposition) [T. Baumgratz et al., Phys. Rev. Lett. 113, 140401 (2014); A. Winter and D. Yang, Phys. Rev. Lett. 116, 120404 ( 2016)] to the setting where not just the free (”incoherent”) resources, but also the manipulated objects, are quantum operations rather than states. In particular, we discuss an information theoretic notion of the coherence capacity of a quantum channel and prove a single-letter formula for it in the case of unitaries. Then we move to the coherence cost of simulating a channel and prove achievability results for unitaries and general channels acting on a d-dimensional system; we show that a maximally coherent state of rank d is always sufficient as a resource if incoherent operations are allowed, and one of rank d(2) for “strictly incoherent” operations. We also show lower bounds on the simulation cost of channels that allow us to conclude that there exists bound coherence in operations, i.e., maps with nonzero cost of implementing them but zero coherence capacity; this is in contrast to states, which do not exhibit bound coherence.

%B PHYSICAL REVIEW A %V 95 %R 10.1103/PhysRevA.95.062327 %0 Journal Article %J JOURNAL OF MATHEMATICAL PHYSICS %D 2017 %T Unitary 2-designs from random X- and Z-diagonal unitaries %A Nakata, Yoshifumi %A Hirche, Christoph %A Morgan, Ciara %A Winter, Andreas %XUnitary 2-designs are random unitaries simulating up to the second order statistical moments of the uniformly distributed random unitaries, often referred to as Haar random unitaries. They are used in a wide variety of theoretical and practical quantum information protocols and also have been used to model the dynamics in complex quantum many-body systems. Here, we show that unitary 2-designs can be approximately implemented by alternately repeating random unitaries diagonal in the Pauli-Z basis and Pauli-X basis. We also provide a converse about the number of repetitions needed to achieve unitary 2-designs. These results imply that the process after l repetitions achieves a Theta(d(-l))-approximate unitary 2-design. Based on the construction, we further provide quantum circuits that efficiently implement approximate unitary 2-designs. Although a more efficient implementation of unitary 2-designs is known, our quantum circuit has its own merit that it is divided into a constant number of commuting parts, which enables us to apply all commuting gates simultaneously and leads to a possible reduction of an actual execution time. We finally interpret the result in terms of the dynamics generated by time-dependent Hamiltonians and provide for the first time a random disordered time-dependent Hamiltonian that generates a unitary 2-design after switching interactions only a few times. Published by AIP Publishing.

%B JOURNAL OF MATHEMATICAL PHYSICS %V 58 %R 10.1063/1.4983266 %0 Journal Article %J Problems of Information Transmission %D 2016 %T Classical capacities of quantum channels with environment assistance %A Karumanchi, S. %A Mancini, S. %A Winter, A. %A Yang, D. %X A quantum channel physically is a unitary interaction between an information carrying system and an environment, which is initialized in a pure state before the interaction. Conventionally, this state, as also the parameters of the interaction, is assumed to be fixed and known to the sender and receiver. Here, following the model introduced by us earlier [1], we consider a benevolent third party, i.e., a helper, controlling the environment state, and show how the helper’s presence changes the communication game. In particular, we define and study the classical capacity of a unitary interaction with helper, in two variants: one where the helper can only prepare separable states across many channel uses, and one without this restriction. Furthermore, two even more powerful scenarios of pre-shared entanglement between helper and receiver, and of classical communication between sender and helper (making them conferencing encoders) are considered. %B Problems of Information Transmission %V 52 %P 214–238 %8 jul %U https://link.springer.com/article/10.1134/S0032946016030029 %R 10.1134/S0032946016030029 %0 Journal Article %J Physical Review Letters %D 2016 %T Entanglement and Coherence in Quantum State Merging %A Streltsov, A. %A Chitambar, E. %A Rana, S. %A Bera, N. %A Winter, A. %A Lewenstein, M. %B Physical Review Letters %V 116 %8 Jan-06-2016 %U http://link.aps.org/doi/10.1103/PhysRevLett.116.240405http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.116.240405/fulltexthttp://link.aps.org/accepted/10.1103/PhysRevLett.116.240405http://link.aps.org/article/10.1103/PhysRevLett.116.240405 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.116.240405 %0 Journal Article %J Journal of Functional Analysis %D 2016 %T Estimating quantum chromatic numbers %A Paulsen, Vern I. %A Severini, Simone %A Stahlke, Daniel %A Todorov, Ivan G. %A Winter, Andreas %B Journal of Functional Analysis %V 270 %P 2188-2222 %8 03/2016 %N 6 %! Journal of Functional Analysis %R 10.1016/j.jfa.2016.01.010 %0 Journal Article %J Nature Communications %D 2016 %T Microcanonical and resource-theoretic derivations of the thermal state of a quantum system with noncommuting charges %A Yunger Halpern, Nicole %A Faist, Philippe %A Oppenheim, Jonathan %A Winter, Andreas %B Nature Communications %V 77 %P 12051 %8 Jul-07-2016 %U http://www.nature.com/doifinder/10.1038/ncomms12051 %! Nat Comms %R 10.1038/ncomms12051 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T No-Signalling-Assisted Zero-Error Capacity of Quantum Channels and an Information Theoretic Interpretation of the Lovász Number %A Duan, Runyao %A Winter, Andreas %B IEEE Transactions on Information Theory %V 62 %P 891-914 %8 2/2016 %N 2 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2015.2507979 %0 Journal Article %J Physical Review Letters %D 2016 %T Operational Resource Theory of Coherence %A Winter, Andreas %A Dong Yang %B Physical Review Letters %V 116 %P 120404 %8 3/2016 %N 12 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.116.120404 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T Polar Codes in Network Quantum Information Theory %A Christoph Hirche %A Ciara Morgan %A Wilde, Mark M. %B IEEE Transactions on Information Theory %V 62 %P 915 - 924 %8 02/2016 %N 2 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2514319 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T Potential Capacities of Quantum Channels %A Winter, Andreas %A Dong Yang %B IEEE Transactions on Information Theory %V 62 %P 1415-1424 %8 3/2016 %N 3 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2519920 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T The Private and Public Correlation Cost of Three Random Variables With Collaboration %A Eric Chitambar %A Hsieh, Min-Hsiu %A Winter, Andreas %B IEEE Transactions on Information Theory %V 62 %P 2034-2043 %8 4/2016 %N 4 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2530086 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T Quantum Channel Capacities With Passive Environment Assistance %A Karumanchi, Siddharth %A Mancini, Stefano %A Winter, Andreas %A Dong Yang %B IEEE Transactions on Information Theory %V 62 %P 1733-1747 %8 4/2016 %N 4 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2016.2522192 %0 Journal Article %J Journal of Mathematical Physics %D 2016 %T Quantum learning of classical stochastic processes: The completely positive realization problem %A Monràs, Alex %A Winter, Andreas %B Journal of Mathematical Physics %V 57 %P 015219 %8 01/2016 %N 1 %! J. Math. Phys. %R 10.1063/1.4936935 %0 Journal Article %J Quantum Science and Technology %D 2016 %T Realising a quantum absorption refrigerator with an atom-cavity system %A Mitchison, Mark T. %A Huber, Marcus %A Prior, Javier %A Woods, Mischa P. %A Plenio, Martin B. %X An autonomous quantum thermal machine comprising a trapped atom or ion placed inside an optical cavity is proposed and analysed. Such a machine can operate as a heat engine whose working medium is the quantised atomic motion or as an absorption refrigerator that cools without any work input. Focusing on the refrigerator mode, we predict that it is possible with state-of-the-art technology to cool a trapped ion almost to its motional ground state using a thermal light source such as sunlight. We nonetheless find that a laser or a similar reference system is necessary to stabilise the cavity frequencies. Furthermore, we establish a direct and heretofore unacknowledged connection between the abstract theory of quantum absorption refrigerators and practical sideband cooling techniques. We also highlight and clarify some assumptions underlying several recent theoretical studies on self-contained quantum engines and refrigerators. Our work indicates that cavity quantum electrodynamics is a promising and versatile experimental platform for the study of autonomous thermal machines in the quantum domain. %B Quantum Science and Technology %V 1 %P 015001 %U http://stacks.iop.org/2058-9565/1/i=1/a=015001 %R 10.1088/2058-9565/1/1/015001 %0 Journal Article %J Physical Review Letters %D 2016 %T Schur Complement Inequalities for Covariance Matrices and Monogamy of Quantum Correlations %A Lami, Ludovico %A Hirche, Christoph %A Adesso, Gerardo %A Winter, Andreas %B Physical Review Letters %V 117 %8 Jan-11-2016 %U http://link.aps.org/doi/10.1103/PhysRevLett.117.220502http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.117.220502/fulltexthttp://link.aps.org/article/10.1103/PhysRevLett.117.220502 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.117.220502 %0 Journal Article %J Physical Review Letters %D 2016 %T Should Entanglement Measures be Monogamous or Faithful? %A Lancien, Cécilia %A Di Martino, Sara %A Huber, Marcus %A Piani, Marco %A Adesso, Gerardo %A Winter, Andreas %B Physical Review Letters %V 117 %8 8/2016 %N 6 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.117.060501 %0 Journal Article %J Communications in Mathematical Physics %D 2016 %T Tight Uniform Continuity Bounds for Quantum Entropies: Conditional Entropy, Relative Entropy Distance and Energy Constraints %A Winter, Andreas %B Communications in Mathematical Physics %V 347 %P 291-313 %8 08/2016 %U http://arxiv.org/abs/1507.07775 %! Commun. Math. Phys. %R 10.1007/s00220-016-2609-8 %0 Journal Article %J Journal of Cryptology %D 2016 %T Weak Locking Capacity of Quantum Channels Can be Much Larger Than Private Capacity %A Winter, Andreas %B Journal of Cryptology %U http://arxiv.org/abs/1403.6361 %! J Cryptol %R 10.1007/s00145-015-9215-3 %0 Journal Article %J New Journal of Physics %D 2016 %T Witnessing entanglement by proxy %A Bäuml, Stefan %A Bruß, Dagmar %A Huber, Marcus %A Kampermann, Hermann %A Winter, Andreas %B New Journal of Physics %V 18 %P 015002 %8 01/2016 %N 1 %! New J. Phys. %R 10.1088/1367-2630/18/1/015002 %0 Journal Article %J IEEE Transactions on Information Theory %D 2016 %T On {Zero}-{Error} {Communication} via {Quantum} {Channels} in the {Presence} of {Noiseless} {Feedback} %A Duan, R. %A Severini, S. %A Winter, A. %K bipartite equivocation graph %K Bipartite graph %K Capacity planning %K Choi-Kraus operators %K coding theorem %K commutative bipartite graph %K Electronic mail %K encoding %K entanglement-assisted capacity %K feedback %K feedback-assisted capacity %K feedback-assisted zero-error capacity %K information theory %K noiseless feedback channel %K quantum channels %K quantum communication %K quantum entanglement %K Quantum information %K Receivers %K Shannon's zero-error communication %K telecommunication channels %K unlimited quantum capacity %K zero-error capacity %X We initiate the study of zero-error communication via quantum channels when the receiver and the sender have at their disposal a noiseless feedback channel of unlimited quantum capacity, generalizing Shannon's zero-error communication theory with instantaneous feedback. We first show that this capacity is only a function of the linear span of Choi-Kraus operators of the channel, which generalizes the bipartite equivocation graph of a classical channel, and which we dub non-commutative bipartite graph. Then, we go on to show that the feedback-assisted capacity is non-zero (allowing for a constant amount of activating noiseless communication) if and only if the non-commutative bipartite graph is non-trivial, and give a number of equivalent characterizations. This result involves a far-reaching extension of the conclusive exclusion of quantum states. We then present an upper bound on the feedback-assisted zero-error capacity, motivated by a conjecture originally made by Shannon and proved later by Ahlswede. We demonstrate that this bound to have many good properties, including being additive and given by a minimax formula. We also prove a coding theorem showing that this quantity is the entanglement-assisted capacity against an adversarially chosen channel from the set of all channels with the same Choi-Kraus span, which can also be interpreted as the feedback-assisted unambiguous capacity. The proof relies on a generalization of the Postselection Lemma (de Finetti reduction) that allows to reflect additional constraints, and which we believe to be of independent interest. This capacity is a relaxation of the feedback-assisted zero-error capacity; however, we have to leave open the question of whether they coincide in general. We illustrate our ideas with a number of examples, including classical-quantum channels and Weyl diagonal channels, and close with an extensive discussion of open questions. %B IEEE Transactions on Information Theory %V 62 %P 5260–5277 %8 sep %R 10.1109/TIT.2016.2562580 %0 Journal Article %J New Journal of Physics %D 2015 %T Coherence-assisted single-shot cooling by quantum absorption refrigerators %A Mitchison, Mark T %A Woods, Mischa P %A Prior, Javier %A Huber, Marcus %B New Journal of Physics %V 17 %P 115013 %G eng %U http://stacks.iop.org/1367-2630/17/i=11/a=115013?key=crossref.3c2d7f1bc5de4bbea1cba38fd2236293 %R 10.1088/1367-2630/17/11/115013 %0 Web Page %D 2015 %T Decoupling with random diagonal-unitary matrices %A Yoshifumi Nakata %A Christoph Hirche %A Ciara Morgan %A Winter, Andreas %U http://arxiv.org/abs/1509.05155 %0 Web Page %D 2015 %T Implementing unitary 2-designs using random diagonal-unitary matrices %A Yoshifumi Nakata %A Christoph Hirche %A Ciara Morgan %A Winter, Andreas %U http://arxiv.org/abs/1502.07514 %0 Journal Article %J Nature Communications %D 2015 %T Limitations on quantum key repeaters %A Bäuml, Stefan %A Christandl, Matthias %A Horodecki, Karol %A Winter, Andreas %B Nature Communications %V 6 %P 6908 %8 4/2015 %! Nat Comms %R 10.1038/ncomms7908 %0 Journal Article %J IEEE Transactions on Information Theory %D 2015 %T Strong Converse for the Classical Capacity of Optical Quantum Communication Channels %A Bardhan, Bhaskar Roy %A Garcia-Patron, Raul %A Wilde, Mark M. %A Winter, Andreas %B IEEE Transactions on Information Theory %V 61 %P 1842-1850 %8 4/2015 %N 4 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2015.2403840 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T Bounds on entanglement assisted source-channel coding via the Lovasz theta number and its variants %A Toby S. Cubitt %A Laura Mancinska %A David Roberson %A Stahlke, Daniel %A Winter, Andreas %B IEEE Transactions on Information Theory %V 60 %P 7330-7344 %8 11/2014 %R 10.1109/TIT.2014.2349502 %0 Conference Proceedings %B 2014 IEEE International Symposium on Information Theory (ISIT) %D 2014 %T Constant compositions in the sphere packing bound for classical-quantum channels %A Dalai, Marco %A Winter, Andreas %B 2014 IEEE International Symposium on Information Theory (ISIT) %I IEEE %C Honolulu, HI, USA %P 151-155 %8 07/2014 %R 10.1109/ISIT.2014.6874813 %0 Journal Article %J Communications in Mathematical Physics %D 2014 %T Everything You Always Wanted to Know About LOCC (But Were Afraid to Ask) %A Eric Chitambar %A Debbie Leung %A Mančinska, Laura %A Maris Ozols %A Winter, Andreas %B Communications in Mathematical Physics %V 328 %P 303-326 %8 5/2014 %N 1 %! Commun. Math. Phys. %R 10.1007/s00220-014-1953-9 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T Full Security of Quantum Key Distribution From No-Signaling Constraints %A Masanes, Lluis %A Renner, Renato %A Christandl, Matthias %A Winter, Andreas %A Barrett, Jonathan %B IEEE Transactions on Information Theory %V 60 %P 4973-4986 %8 8/2014 %N 8 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2014.2329417 %0 Journal Article %J Physical Review Letters %D 2014 %T Graph-Theoretic Approach to Quantum Correlations %A Cabello, Adán %A Severini, Simone %A Winter, Andreas %B Physical Review Letters %V 112 %P 040401 %8 1/2014 %N 4 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.112.040401 %0 Journal Article %J Linear Algebra and its Applications %D 2014 %T Inequalities for the ranks of multipartite quantum states %A Josh Cadney %A Huber, Marcus %A Noah Linden %A Winter, Andreas %X We investigate relations between the ranks of marginals of multipartite quantum states. These are the Schmidt ranks across all possible bipartitions and constitute a natural quantification of multipartite entanglement dimensionality. We show that there exist inequalities constraining the possible distribution of ranks. This is analogous to the case of von Neumann entropy (\alpha-R\'enyi entropy for \alpha=1), where nontrivial inequalities constraining the distribution of entropies (such as e.g. strong subadditivity) are known. It was also recently discovered that all other \alpha-R\'enyi entropies for α∈(0,1)∪(1,∞) satisfy only one trivial linear inequality (non-negativity) and the distribution of entropies for α∈(0,1) is completely unconstrained beyond non-negativity. Our result resolves an important open question by showing that also the case of \alpha=0 (logarithm of the rank) is restricted by nontrivial linear relations and thus the cases of von Neumann entropy (i.e., \alpha=1) and 0-R\'enyi entropy are exceptionally interesting measures of entanglement in the multipartite setting. %B Linear Algebra and its Applications %V 452 %P 153 - 171 %8 07/2014 %! Linear Algebra and its Applications %R 10.1016/j.laa.2014.03.035 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T ‘Pretty strong’ converse for the quantum capacity of degradable channels %A Ciara Morgan %A Winter, Andreas %B IEEE Transactions on Information Theory %V 60 %P 317-333 %8 01/2014 %R 10.1109/TIT.2013.2288971 %0 Journal Article %J IEEE Transactions on Information Theory %D 2014 %T The Quantum Reverse Shannon Theorem and Resource Tradeoffs for Simulating Quantum Channels %A Bennett, Charles H. %A Devetak, Igor %A Harrow, Aram W. %A Shor, Peter W. %A Winter, Andreas %B IEEE Transactions on Information Theory %V 60 %P 2926-2959 %8 5/2014 %N 5 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2014.2309968 %0 Journal Article %J Communications in Mathematical Physics %D 2014 %T Relative Entropy and Squashed Entanglement %A Ke Li %A Winter, Andreas %B Communications in Mathematical Physics %V 326 %P 63-80 %8 2/2014 %N 1 %! Commun. Math. Phys. %R 10.1007/s00220-013-1871-2 %0 Journal Article %J Communications in Mathematical Physics %D 2014 %T Strong Converse for the Classical Capacity of Entanglement-Breaking and Hadamard Channels via a Sandwiched Rényi Relative Entropy %A Wilde, Mark M. %A Winter, Andreas %A Dong Yang %B Communications in Mathematical Physics %V 331 %P 593-622 %8 10/2014 %N 2 %! Commun. Math. Phys. %R 10.1007/s00220-014-2122-x %0 Journal Article %J Problems of Information Transmission %D 2014 %T Strong converse for the classical capacity of the pure-loss bosonic channel %A Wilde, Mark M. %A Winter, Andreas %B Problems of Information Transmission %V 50 %P 117-132 %8 4/2014 %N 2 %! Probl Inf Transm %R 10.1134/S003294601402001X %0 Conference Proceedings %B 9th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2014) %D 2014 %T Strong Converse for the Quantum Capacity of the Erasure Channel for Almost All Codes %A Wilde, Mark M. %A Winter, Andreas %E Steve T Flammia %E Harrow, Aram W. %B 9th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2014) %I LIPICS %C Singapore %V 27 %P 52-66 %R 10.4230/LIPIcs.TQC.2014.52 %0 Journal Article %J Physical Review A %D 2014 %T Unified approach to entanglement criteria using the Cauchy-Schwarz and Hölder inequalities %A Wölk, Sabine %A Huber, Marcus %A Gühne, Otfried %X We present a unified approach to different recent entanglement criteria. Although they were developed in different ways, we show that they are all applications of a more general principle given by the Cauchy-Schwarz inequality. We explain this general principle and show how to use it to derive not only already known entanglement criteria, but also criteria which were unknown so far. We systematically investigate its potential and limitations to detect bipartite and multipartite entanglement. Furthermore, we describe how to apply our generalized entanglement detection scheme to find the measurement directions to verify the entanglement of a given state experimentally. %B Physical Review A %V 90 %8 8/2014 %N 2 %! Phys. Rev. A %R 10.1103/PhysRevA.90.022315 %0 Journal Article %J Journal of Physics A: Mathematical and Theoretical %D 2014 %T What does an experimental test of quantum contextuality prove or disprove? %A Winter, Andreas %B Journal of Physics A: Mathematical and Theoretical %V 47 %P 424031 %8 10/2014 %N 42 %! J. Phys. A: Math. Theor. %R 10.1088/1751-8113/47/42/424031 %0 Journal Article %J Physical Review Letters %D 2013 %T Bose-Glass Phases of Ultracold Atoms due to Cavity Backaction %A Hessam Habibian %A André Winter %A Paganelli, Simone %A Heiko Rieger %A Giovanna Morigi %X We determine the quantum ground-state properties of ultracold bosonic atoms interacting with the mode of a high-finesse resonator. The atoms are confined by an external optical lattice, whose period is incommensurate with the cavity mode wavelength, and are driven by a transverse laser, which is resonant with the cavity mode. While for pointlike atoms photon scattering into the cavity is suppressed, for sufficiently strong lasers quantum fluctuations can support the buildup of an intracavity field, which in turn amplifies quantum fluctuations. The dynamics is described by a Bose-Hubbard model where the coefficients due to the cavity field depend on the atomic density at all lattice sites. Quantum Monte Carlo simulations and mean-field calculations show that, for large parameter regions, cavity backaction forces the atoms into clusters with a checkerboard density distribution. Here, the ground state lacks superfluidity and possesses finite compressibility, typical of a Bose glass. This system constitutes a novel setting where quantum fluctuations give rise to effects usually associated with disorder. %B Physical Review Letters %V 110 %8 2/2013 %U http://prl.aps.org/abstract/PRL/v110/i7/e075304 %N 7 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.110.075304 %0 Journal Article %J Communications in Mathematical Physics %D 2013 %T Distinguishing Multi-Partite States by Local Measurements %A Lancien, Cécilia %A Winter, Andreas %B Communications in Mathematical Physics %V 323 %P 555 - 573 %8 10/2013 %N 2 %! Commun. Math. Phys. %R 10.1007/s00220-013-1779-x %0 Book Section %B Information Theory, Combinatorics, and Search Theory: In Memory of Rudolf Ahlswede %D 2013 %T Identification via Quantum Channels %A Winter, Andreas %E Aydinian, Harout %E Cicalese, Ferdinando %E Deppe, Christian %B Information Theory, Combinatorics, and Search Theory: In Memory of Rudolf Ahlswede %S Lecture Notes in Computer Science %I Springer Berlin Heidelberg %C Berlin, Heidelberg %V LNCS 7777 %P 217 - 233 %@ 978-3-642-36899-8 %R 10.1007/978-3-642-36899-8_9 %0 Conference Proceedings %B 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) %D 2013 %T The Quantum Entropy Cone of Stabiliser States %A Noah Linden %A Frantisek Matus %A Mary Beth Ruskai %A Winter, Andreas %E Severini, Simone %E Fernando Brandao %B 8th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2013) %S LIPICS %C Guelph, ON %V 22 %P 270-284 %8 02/2013 %R 10.4230/LIPIcs.TQC.2013.270 %0 Journal Article %J Phyiscal Review A %D 2013 %T Quantum phases of incommensurate optical lattices due to cavity backaction %A Hessam Habibian %A André Winter %A Paganelli, Simone %A Heiko Rieger %A Giovanna Morigi %B Phyiscal Review A %V 88 %P 043618 %0 Journal Article %J IEEE Transactions on Information Theory %D 2013 %T Quantum Rate-Distortion Coding With Auxiliary Resources %A Wilde, Mark M. %A Datta, Nilanjana %A Hsieh, Min-Hsiu %A Winter, Andreas %B IEEE Transactions on Information Theory %V 59 %P 6755 - 6773 %8 10/2013 %N 10 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2013.2271772 %0 Journal Article %J Physical Review Letters %D 2013 %T Quantum State Cloning Using Deutschian Closed Timelike Curves %A Brun, Todd A. %A Wilde, Mark M. %A Winter, Andreas %B Physical Review Letters %V 111 %P 190401 %8 11/2013 %N 19 %! Phys. Rev. Lett. %R 10.1103/PhysRevLett.111.190401 %0 Journal Article %J Journal of Mathematical Physics %D 2013 %T Quantum-to-classical rate distortion coding %A Datta, Nilanjana %A Hsieh, Min-Hsiu %A Wilde, Mark M. %A Winter, Andreas %B Journal of Mathematical Physics %V 54 %P 042201 %8 2013 %N 4 %! J. Math. Phys. %R 10.1063/1.4798396 %0 Journal Article %J Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences %D 2013 %T The structure of Renyi entropic inequalities %A Linden, N. %A Mosonyi, M. %A Winter, A. %B Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences %V 469 %P 20120737 %8 10/2013 %N 2158472222 %! Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences %R 10.1098/rspa.2012.0737 %0 Journal Article %J IEEE Transactions on Information Theory %D 2013 %T Zero-Error Communication via Quantum Channels, Noncommutative Graphs, and a Quantum Lovász Number %A Duan, Runyao %A Severini, Simone %A Winter, Andreas %B IEEE Transactions on Information Theory %V 59 %P 1164 - 1174 %8 02/2013 %N 2 %! IEEE Trans. Inform. Theory %R 10.1109/TIT.2012.2221677 %0 Journal Article %J New J. Phys. %D 2012 %T Glass to superfluid transition in dirty bosons on a lattice %A Julia Stasińska %A Massignan, Pietro %A Bishop, Michael %A Wehr, Jan %A Sanpera, Anna %A Lewenstein, Maciej %X We investigate the interplay between disorder and interactions in a Bose gas on a lattice in presence of randomly localized impurities. We compare the performance of two theoretical methods, the simple version of multi-orbital Hartree-Fock and the common Gross-Pitaevskii approach, showing how the former gives a very good approximation to the ground state in the limit of weak interactions, where the superfluid fraction is small. We further prove rigorously that for this class of disorder the fractal dimension of the ground state d* tends to the physical dimension in the thermodynamic limit. This allows us to introduce a quantity, the fractional occupation, which gives insightful information on the crossover from a Lifshits to a Bose glass. Finally, we compare temperature and interaction effects, highlighting similarities and intrinsic differences. %B New J. Phys. %V 14 %P 043043 %U http://arxiv.org/abs/1111.3494 %R 10.1088/1367-2630/14/4/043043 %0 Journal Article %J Phys. Rev. Lett. %D 2011 %T All Nonclassical Correlations Can Be Activated into Distillable Entanglement %A Piani, Marco %A Gharibian, Sevag %A Adesso, Gerardo %A Calsamiglia, John %A Horodecki, Paweł %A Winter, Andreas %B Phys. Rev. Lett. %I American Physical Society %V 106 %P 220403 %8 Jun %U http://prl.aps.org/abstract/PRL/v106/i22/e220403 %) http://arxiv.org/abs/1103.4032 %R 10.1103/PhysRevLett.106.220403 %0 Journal Article %J EPL (Europhysics Letters) %D 2011 %T Entangling two distant oscillators with a quantum reservoir %A Wolf, A %A De Chiara, G. %A Kajari, E %A Lutz, E %A G. Morigi %X The generation of entanglement between two oscillators that interact via a common reservoir is theoretically studied. The reservoir is modeled by a one-dimensional harmonic crystal initially in thermal equilibrium. Starting from a separable state, the oscillators can become entangled after a transient time, that is of the order of the thermalization time scale. This behaviour is observed at finite temperature even when the oscillators are at a distance significantly larger than the crystal's interparticle spacing. The underlying physical mechanisms can be explained by the dynamical properties of the collective variables of the two oscillators which may decouple from or be squeezed by the reservoir. Our predictions can be tested with an ion chain in a linear Paul trap. %B EPL (Europhysics Letters) %V 95 %P 60008 %8 09/2011 %U http://epljournal.edpsciences.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/epl/abs/2011/18/epl13816/epl13816.html %N 6 %! EPL %R 10.1209/0295-5075/95/60008 %0 Journal Article %J Physical Review Letters %D 2007 %T Trapped Ion Chain as a Neural Network: Error Resistant Quantum Computation %A Pons, Marisa %A Ahufinger, Veronica %A Wunderlich, Christof %A Sanpera, Anna %A Braungardt, Sibylle %A Sen(De), Aditi %A Sen, Ujjwal %A Lewenstein, Maciej %K neural networks %K quantum computation %B Physical Review Letters %V 98 %P 023003–4 %8 01/2007 %G eng %U http://link.aps.org/abstract/PRL/v98/e023003 %R 10.1103/PhysRevLett.98.023003 %0 Journal Article %J Physical Review Letters %D 2004 %T Experimental Detection of Multipartite Entanglement using Witness Operators %A Bourennane, Mohamed %A Eibl, Manfred %A Kurtsiefer, Christian %A Gaertner, Sascha %A Weinfurter, Harald %A Gühne, Otfried %A Hyllus, Philipp %A Bruß, Dagmar %A Lewenstein, Maciej %A Sanpera, Anna %B Physical Review Letters %V 92 %G eng %U http://prl.aps.org/abstract/PRL/v92/i8/e087902 %R 10.1103/PhysRevLett.92.087902 %0 Journal Article %J Science (New York, N.Y.) %D 2001 %T Feynman's path-integral approach for intense-laser-atom interactions %A Salieres, P. %A Carre, B. %A Le Deroff, L. %A Grasbon, F. %A Paulus, G. G. %A Walther, H. %A Kopold, R. %A Becker, W. %A Milosevic, D. B. %A Sanpera, A. %A Lewenstein, M. %X Atoms interacting with intense laser fields can emit electrons and photons of very high energies. An intuitive and quantitative explanation of these highly nonlinear processes can be found in terms of a generalization of classical Newtonian particle trajectories, the so-called quantum orbits. Very few quantum orbits are necessary to reproduce the experimental results. These orbits are clearly identified, thus opening the way for an efficient control as well as previously unknown applications of these processes. %B Science (New York, N.Y.) %V 292 %P 902–5 %G eng %U http://www.sciencemag.org/cgi/content/abstract/292/5518/902 %R 10.1126/science.108836 %0 Conference Paper %B {2018 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY (ISIT)} %D 0 %T {Fully Quantum Arbitrarily Varying Channels: Random Coding Capacity and Capacity Dichotomy} %A Boche, Holger %A Deppe, Christian %A Noetzel, Janis %A Winter, Andreas %X {We consider a model of communication via a fully quantum jammer channel with quantum jammer, quantum sender and quantum receiver, which we dub quantum arbitrarily varying channel (QAVC). Restricting to finite dimensional user and jammer systems, we show, using permutation symmetry and a de Finetti reduction, how the random coding capacity (classical and quantum) of the QAVC is reduced to the capacity of a naturally associated compound channel, which is obtained by restricting the jammer to i.i.d. input states. Furthermore, we demonstrate that the shared randomness required is at most logarithmic in the block length, via a quantum version of the ``elimination of of correlation{''} using a random matrix tail bound. This implies a dichotomy theorem: either the classical capacity of the QAVC is zero, and then also the quantum capacity is zero, or each capacity equals its random coding variant.} %B {2018 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY (ISIT)} %S {IEEE International Symposium on Information Theory} %I {IEEE; IEEE Informat Theory Soc; NSF; Huawei; Qualcomm} %P {2012-2016} %@ {978-1-5386-4781-3} %0 Journal Article %J {NATURE COMMUNICATIONS} %D 0 %T {Generalized laws of thermodynamics in the presence of correlations} %A Bera, Manabendra N. %A Riera, Arnau %A Lewenstein, Maciej %A Winter, Andreas %X {The laws of thermodynamics, despite their wide range of applicability, are known to break down when systems are correlated with their environments. Here we generalize thermodynamics to physical scenarios which allow presence of correlations, including those where strong correlations are present. We exploit the connection between information and physics, and introduce a consistent redefinition of heat dissipation by systematically accounting for the information flow from system to bath in terms of the conditional entropy. As a consequence, the formula for the Helmholtz free energy is accordingly modified. Such a remedy not only fixes the apparent violations of Landauer's erasure principle and the second law due to anomalous heat flows, but also leads to a generally valid reformulation of the laws of thermodynamics. In this information-theoretic approach, correlations between system and environment store work potential. Thus, in this view, the apparent anomalous heat flows are the refrigeration processes driven by such potentials.} %B {NATURE COMMUNICATIONS} %V {8} %8 {DEC 19} %R {10.1038/s41467-017-02370-x} %0 Journal Article %J {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES} %D 0 %T {Quantum reference frames and their applications to thermodynamics} %A Popescu, Sandu %A Belen Sainz, Ana %A Short, Anthony J. %A Winter, Andreas %X{We construct a quantum reference frame, which can be used to approximately implement arbitrary unitary transformations on a system in the presence of any number of extensive conserved quantities, by absorbing any back action provided by the conservation laws. Thus, the reference frame at the same time acts as a battery for the conserved quantities. Our construction features a physically intuitive, clear and implementation-friendly realization. Indeed, the reference system is composed of the same types of subsystems as the original system and is finite for any desired accuracy. In addition, the interaction with the reference frame can be broken down into two-body terms coupling the system to one of the reference frame subsystems at a time. We apply this construction to quantum thermodynamic set-ups with multiple, possibly non-commuting conserved quantities, which allows for the definition of explicit batteries in such cases. This article is part of a discussion meeting issue `Foundations of quantum mechanics and their impact on contemporary society'.}

%B {PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES} %V {376} %8 {JUL 13} %R {10.1098/rsta.2018.0111} %0 Conference Paper %B {2018 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY (ISIT)} %D 0 %T {Secure and Robust Identification via Classical-Quantum Channels} %A Boche, Holger %A Deppe, Christian %A Winter, Andreas %X {We study the identification capacity of classical-quantum channels ({''}cq-channels{''}), under channel uncertainty and privacy constraints. To be precise, we consider first compound memoryless cq-channels and determine their identification capacity; then we add an eavesdropper, considering compound memoryless wiretap cqq-channels, and determine their secret identification capacity. In the first case (without privacy), we find the identification capacity always equal to the transmission capacity. In the second case, we find a dichotomy: either the secrecy capacity (also known as private capacity) of the channel is zero, and then also the secrecy identification capacity is zero, or the secrecy capacity is positive and then the secrecy identification capacity equals the transmission capacity of the main channel without the wiretapper. We perform the same analysis for the case of arbitrarily varying wiretap cqq-channels (cqq-AVWC), with analogous findings, and make several observations regarding the continuity and super-additivity of the identification capacity in the latter case.} %B {2018 IEEE INTERNATIONAL SYMPOSIUM ON INFORMATION THEORY (ISIT)} %S {IEEE International Symposium on Information Theory} %I {IEEE; IEEE Informat Theory Soc; NSF; Huawei; Qualcomm} %P {2674-2678} %@ {978-1-5386-4781-3}