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 Paul Skrzypczyk : Research - Quantum non-locality


My research interests cover a few areas, namely

Quantum non-locality: Trying to understand the non-local correlations which are exhibited by measurement outcomes made on entangled quantum states.

Thermodynamics of small systems: Understand what concepts from thermodynamics mean, and how they look, in the quantum regime.

Quantum behaviour: Using the ideas of Modular variables, pre- and post-selection and weak values to better understand systems exhibiting truly quantum behaviour.

My work to date can be found on

My supervisor is Sandu Popescu.

Bound non-locality and activation

Nicolas Brunner, Daniel Cavalcanti, Alejo Salles, Paul Skrzypczyk

We investigate non-locality distillation using measures of non-locality based on the Elitzur-Popescu-Rohrlich decomposition. For a certain number of copies of a given non-local correlation, we define two quantities of interest: (i) the non-local cost, and (ii) the distillable non-locality. We find that there exist correlations whose distillable non-locality is strictly smaller than their non-local cost. Thus non-locality displays a form of irreversibility which we term bound non-locality. Finally we show that non-local distillability can be activated. 

On the efficiency of very small refrigerators

Paul Skrzypczyk, Nicolas Brunner, Noah Linden, Sandu Popescu

We investigate whether size imposes a fundamental constraint on the efficiency of small thermal machines. We analyse in detail a model of a small self-contained refrigerator consisting of three qubits. We show that this system can reach the Carnot efficiency, and thus demonstrate that there exists no complementarity between size and efficiency. 

Physics within a quantum reference frame

Renato M. Angelo, Nicolas Brunner, Sandu Popescu, Antony J. Short, Paul Skrzypczyk

We investigate the physics of quantum reference frames. Specifically, we study several simple scenarios involving a small number of quantum particles, whereby we promote one of these particles to the role of a quantum observer and ask what is the description of the rest of the system, as seen by this observer? We highlight the interesting aspects of such questions by presenting a number of apparent paradoxes. By unraveling these paradoxes we get a better understanding of the physics of quantum reference frames. 

How small can thermal machines be? Towards the smallest possible refrigerator

Noah Linden, Sandu Popescu and Paul Skrzypczyk
Phys. Rev. Lett 105, 130401 (2010)

We investigate the fundamental dimensional limits to thermodynamic machines. In particular, we show that it is possible to construct self-contained refrigerators (i.e., not requiring external sources of work) consisting of only a small number of qubits and/or qutrits. We present three different models, consisting of two qubits, a qubit and a qutrit with nearest-neighbor interactions, and a single qutrit, respectively. We then investigate the fundamental limits to their performance; in particular, we show that it is possible to cool towards absolute zero.

Closed sets of non-local correlations

Jonathan Allcock, Nicolas Brunner, Noah Linden, Sandu Popescu, Paul Skrzypczyk and Tamas Vertesi
Phys. Rev. A 80, 062107 (2009)

In this work we introduce a fundamental concept -- closed sets of correlations -- for studying non-local correlations. We argue that sets of correlations corresponding to information-theoretic principles, or more generally to consistent physical theories, must be closed under a natural set of operations. Hence, studying the closure of sets of correlations gives insight into which information-theoretic principles are genuinely different, and which are ultimately equivalent. This concept also has implications for understanding why quantum non-locality is limited, and for finding constraints on physical theories beyond quantum mechanics. 

Non-locality distillation and post-quantum theories with trivial communication complexity

Nicolas Brunner and Paul Skrzypczyk
Phys. Rev. Lett. 102, 160403 (2009)

Here we first present a protocol for deterministically distilling non-locality, building upon a recent result of Forster et al. [Phys. Rev. Lett. 102, 120401 (2009)]. The protocol works efficiently for a specific class of post-quantum non-local boxes, which we term correlated non-local boxes. In the asymptotic limit, all correlated non-local boxes are distilled to the maximally non-local box of Popescu and Rohrlich. Then, taking advantage of a result of Brassard et al. [Phys. Rev. Lett. 96, 250401 (2006)] we show that all correlated non-local boxes make communication complexity trivial, and therefore appear very unlikely to exist in nature. Some of these non-local boxes are arbitrarily close to the set of classical correlations. 

Couplers for Non-Locality Swapping

Paul Skrzypczyk and Nicolas Brunner
New J. Phys. 11, 073014 (2009)

Studying generalized non-local theories brings insight to the foundations of quantum mechanics. In this work we focus on non-locality swapping, the analogue of quantum entanglement swapping. In order to implement such a protocol, one needs a coupler that performs the equivalent of quantum joint measurements on generalized `box-like' states. By establishing a connection to Bell inequalities, we define consistent couplers for theories containing an arbitrary amount of non-locality, which leads us to introduce the concepts of perfect and minimal couplers. Surprisingly, Tsirelson's bound for quantum non-locality naturally appears in this study. 

Emergence of Quantum Correlations from Non-Locality Swapping

Paul Skrzypczyk, Nicolas Brunner and Sandu Popescu
Phys. Rev. Lett. 102, 110402 (2009)

By studying generalized non-signalling theories, the hope is to find out what makes quantum mechanics so special. In this work we revisit the paradigmatic model of non-signalling boxes and introduce the concept of a genuine box. This allows us to present the first generalized non-signalling model featuring quantum-like dynamics. In particular, we present the coupler, a device enabling non-locality swapping, the analogue of quantum entanglement swapping, as well as teleportation. Interestingly, part of the boundary between quantum and post-quantum correlations emerges from this study.