Publications (23)69.44 Total impact
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ABSTRACT: Genuine randomness can be certified from Bell tests without any detailed assumptions on the working of the devices with which the test is implemented. An important class of experiments for implementing such tests is optical setups based on polarisation measurements of entangled photons distributed from a spontaneous parametric down conversion source. Here we compute the maximal amount of randomness which can be certified in such setups under realistic conditions. We provide relevant yet unexpected numerical values for the physical parameters and achieve four times more randomness than previous methods.10/2014;  [Show abstract] [Hide abstract]
ABSTRACT: The concept of bilocality was introduced to study the correlations which arise in an entanglement swapping scenario, where one has two sources which can naturally taken to be independent. This additional constraint leads to stricter requirements than simply imposing locality, in the form of bilocality inequalities. In this work we consider a natural generalisation of the bilocality scenario, namely the starnetwork consisting of a single central party surrounded by $n$ edge parties, each of which shares an independent source with the centre. We derive new inequalities which are satisfied by all local correlations in this scenario, for the cases when the central party performs (i) two dichotomic measurements (ii) a single Bell state measurement. We demonstrate quantum violations of these inequalities and study both the robustness to noise and to losses.09/2014; 
Article: Extracting work from correlations
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ABSTRACT: We consider the problem of extracting work from isolated quantum systems composed of $n$ subsystems. In this scenario the work can be naturally divided into two contributions: a local contribution from each subsystem, and a global contribution originating from correlations between subsystems. Here we focus on the latter and consider quantum systems which are locally thermal, thus from which work can only be extracted from correlations. We derive bounds on the extractable work for general quantum states, separable states, and states with fixed entropy. Our results show that while entanglement gives an advantage for small quantum systems, this gain vanishes for a large number of subsystems.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: EinsteinPodolskyRosen steering is a form of bipartite quantum correlation that is intermediate between entanglement and Bell nonlocality. It allows for entanglement certification when the measurements performed by one of the parties are not characterized (or are untrusted) and has applications in quantum key distribution. Despite its foundational and applied importance, EinsteinPodolskyRosen steering lacks a quantitative assessment. Here we propose a way of quantifying this phenomenon and use it to study the steerability of several quantum states. In particular, we show that every pure entangled state is maximally steerable and the projector onto the antisymmetric subspace is maximally steerable for all dimensions; we provide a new example of oneway steering and give strong support that states with positivepartial transposition are not steerable.Physical Review Letters 05/2014; 112(18):180404. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the fundamental limitations imposed by thermodynamics for creating correlations. Considering a collection of initially uncorrelated thermal quantum systems, we ask how much classical and quantum correlations can be obtained via a cyclic Hamiltonian process. We derive bounds on both the mutual information and entanglement of formation, as a function of the temperature of the systems and the available energy. We also characterize the maximal temperature that allows for the creation of entanglement. In the multipartite case, we consider several types of entanglement, in particular genuine multipartite and bipartite entanglement. This approach may find applications, e.g. in quantum information processing, for physical platforms in which thermodynamical considerations cannot be ignored.04/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Small selfcontained quantum thermal machines function without external source of work or control but using only incoherent interactions with thermal baths. Here we investigate the role of entanglement in a small selfcontained quantum refrigerator. We first show that entanglement is detrimental as far as efficiency is concernedfridges operating at efficiencies close to the Carnot limit do not feature any entanglement. Moving away from the Carnot regime, we show that entanglement can enhance cooling and energy transport. Hence, a truly quantum refrigerator can outperform a classical one. Furthermore, the amount of entanglement alone quantifies the enhancement in cooling.Physical Review E 03/2014; 89(31):032115. · 2.31 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the problem of giving a model independent definition of the dimension of physical systems. We give two definitions of dimension, based on measurements and on the capacity of storing information. While both definitions coincide in classical and quantum mechanics, they are in general different in generalized probabilistic theories. We discuss in detail the case of a theory known as 'boxworld', and show that such a theory features systems with a dimension mismatch. This dimension mismatch can be made arbitrarily large by using an amplification procedure. Furthermore, we show that the dimension mismatch of this model has strong consequences on its power for performing informationtheoretic tasks, leading to the collapse of communication complexity and to the violation of information causality. Finally, we discuss the consequences of a dimension mismatch from the perspective of thermodynamics, and ask whether this effect could break Landauer's erasure principle and thus the second law.01/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Thermodynamics is traditionally concerned with systems comprised of a large number of particles. Here we present a framework for extending thermodynamics to individual quantum systems, including explicitly a thermal bath and workstorage device (essentially a 'weight' that can be raised or lowered). We prove that the second law of thermodynamics holds in our framework, and gives a simple protocol to extract the optimal amount of work from the system, equal to its change in free energy. Our results apply to any quantum system in an arbitrary initial state, in particular including nonequilibrium situations. The optimal protocol is essentially reversible, similar to classical Carnot cycles, and indeed, we show that it can be used to construct a quantum Carnot engine.Nature Communications 01/2014; 5:4185. · 10.74 Impact Factor 
Article: Quantifying quantum steering
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ABSTRACT: Quantum steering is a form of bipartite quantum correlations that is intermediate between entanglement and Bell nonlocality. It allows for entanglement certification when the measurements performed by one of the parties are not characterised (or untrusted) and has applications in quantum key distribution. Despite its foundational and applied importance, quantum steering lacks a quantitative assessment. Here we propose a way of quantifying this phenomenon and study the steering power of several quantum states. In particular we show that every pure entangled state is maximally steerable. Furthermore we study the steering power of several interesting states, and give strong support that states with positivepartialtransposition are not steerable.11/2013; 
Article: Extracting work from quantum systems
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ABSTRACT: We consider the task of extracting work from quantum systems in the resource theory perspective of thermodynamics, where free states are arbitrary thermal states, and allowed operations are energy conserving unitary transformations. Taking as our work storage system a 'weight' we prove the second law and then present simple protocols which extract average work equal to the free energy change of the system  the same amount as in classical thermodynamics. Crucially, for systems in 'classical' states (mixtures of energy eigenstates) our protocol works on a single copy of the system. This is in sharp contrast to previous results, which showed that in case of almostdeterministic work extraction, collective actions on multiple copies are necessary to extract the free energy. This establishes the fact that free energy is a meaningful notion even for individual systems in classical states. However, for nonclassical states, where coherences between energy levels exist, we prove that collective actions are necessary, so long as no external sources of coherence are used.02/2013; 
Article: Quantum Cheshire Cats
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ABSTRACT: In this paper we present a quantum Cheshire cat. In a pre and postselected experiment we find the cat in one place, and the smile in another. The cat is a photon, while the smile is it's circular polarisation.New Journal of Physics 02/2012; · 4.06 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: When separated measurements on entangled quantum systems are performed, the theory predicts correlations that cannot be explained by any classical mechanism: communication is excluded because the signal should travel faster than light; preestablished agreement is excluded because Bell inequalities are violated. All optical demonstrations of such violations have involved discrete degrees of freedom and are plagued by the detectionefficiency loophole. A promising alternative is to use continuous variables combined with highly efficient homodyne measurements. However, all the schemes proposed so far use states or measurements that are extremely difficult to achieve, or they produce very weak violations. We present a simple method to generate large violations for feasible states using both photon counting and homodyne detections. The present scheme can also be used to obtain nonlocality from easytoprepare Gaussian states (e.g., twomode squeezed state).Physical Review A 08/2011; 84(2). · 3.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We argue that thermal machines can be understood from the perspective of `virtual qubits' at `virtual temperatures': The relevant way to view the two heat baths which drive a thermal machine is as a composite system. Virtual qubits are twolevel subsystems of this composite, and their virtual temperatures can take on any value, positive or negative. Thermal machines act upon an external system by placing it in thermal contact with a wellselected range of virtual qubits and temperatures. We demonstrate these claims by studying the smallest thermal machines. We show further that this perspective provides a powerful way to view thermodynamics, by analysing a number of phenomena. This includes approaching Carnot efficiency (where we find that all machines do so essentially by becoming equivalent to the smallest thermal machines), entropy production in irreversible machines, and a way to view work in terms of negative temperature and population inversion. Moreover we introduce the idea of "genuine" thermal machines and are led to considering the concept of "strength" of work.Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics 06/2011;  [Show abstract] [Hide abstract]
ABSTRACT: So far, all the optical demonstrations of violations of Bell's inequalities have involved discrete degrees of freedom (e.g. polarization, timebins) and are plagued by the detectionefficiency loophole. Continuous degrees of freedom would be a very interesting alternative because of the efficiency of the homodyne measurement; but the feasible schemes proposed so far reach very weak violations. We show that large violations for easilyprepared states can be achieved if both photon counting and homodyne detections are used. Our simple scheme may lead to the first violation of Bell's inequalities with continuous variables and pave the way for a loopholefree Bell test.03/2011; 
Article: Bound nonlocality and activation.
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ABSTRACT: We investigate nonlocality distillation using measures of nonlocality based on the ElitzurPopescuRohrlich decomposition. For a certain number of copies of a given nonlocal box, we define two quantities of interest: (i) the nonlocal cost and (ii) the distillable nonlocality. We find that there exist boxes whose distillable nonlocality is strictly smaller than their nonlocal cost. Thus nonlocality displays a form of irreversibility which we term "bound nonlocality." Finally, we show that nonlocal distillability can be activated.Physical Review Letters 01/2011; 106(2):020402. · 7.73 Impact Factor 
Article: The smallest possible heat engines
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ABSTRACT: We construct the smallest possible self contained heat engines; one composed of only two qubits, the other of only a single qutrit. The engines are selfcontained as they do not require external sources of work and/or control. They are able to produce work which is used to continuously lift a weight. Despite the dimension of the engine being small, it is still able to operate at the Carnot efficiency. Comment: 5 pages, 2 figures10/2010;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate whether size imposes a fundamental constraint on the efficiency of small thermal machines. We analyse in detail a model of a small selfcontained refrigerator consisting of three qubits. We show analytically that this system can reach the Carnot efficiency, thus demonstrating that there exists no complementarity between size and efficiency.Journal of Physics A Mathematical and Theoretical 09/2010; · 1.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the fundamental dimensional limits to thermodynamic machines. In particular, we show that it is possible to construct selfcontained 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 nearestneighbor 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.Physical Review Letters 09/2010; 105(13):130401. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: 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 unravelling these paradoxes we get a better understanding of the physics of quantum reference frames.Journal of Physics A Mathematical and Theoretical 07/2010; 44(14). · 1.77 Impact Factor 
Article: Closed sets of nonlocal correlations
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ABSTRACT: We present a fundamental concept—closed sets of correlations—for studying nonlocal correlations. We argue that sets of correlations corresponding to informationtheoretic 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 informationtheoretic principles are genuinely different, and which are ultimately equivalent. This concept also has implications for understanding why quantum nonlocality is limited, and for finding constraints on physical theories beyond quantum mechanics.Physical Review A 08/2009; 80(6). · 3.04 Impact Factor
Publication Stats
157  Citations  
69.44  Total Impact Points  
Top Journals
Institutions

2014

ICFO Institute of Photonic Sciences
Barcino, Catalonia, Spain


2009–2011

University of Bristol
 School of Mathematics
Bristol, England, United Kingdom
