Publications (31)87.08 Total impact

Article: Postquantum steering
[Show abstract] [Hide abstract]
ABSTRACT: The discovery of postquantum nonlocality, i.e. the existence of nonlocal correlations stronger than any quantum correlations but nevertheless consistent with the nosignaling principle, has deepened our understanding of the foundations quantum theory. In this work, we investigate whether the phenomenon of EinsteinPodolskyRosen steering, a different form of quantum nonlocality, can also be generalized beyond quantum theory. While postquantum steering does not exist in the bipartite case, we prove its existence in the case of three observers. Importantly, we show that postquantum steering is a genuinely new phenomenon, fundamentally different from postquantum nonlocality. Our results provide new insight into the nonlocal correlations of multipartite quantum systems.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum mechanics predicts the existence of intrinsically random processes. Contrary to classical randomness, this lack of predictability can not be attributed to ignorance or lack of control. Here we propose a method to quantify the amount of randomness that can be extracted in two scenarios: (i) the quantum steering scenario, where two parties measure a bipartite system in an unknown state but one of them does not trust his measurement apparatus, and (ii) the prepareandmeasure scenario, where additionally the quantum state is known. We use our methods to compute the maximal amount of local randomness that can be certified by measuring systems subject to noise and losses and show that randomness can be certified from a single measurement if and only if the detectors used in the test have detection efficiency higher than 50%.  [Show abstract] [Hide abstract]
ABSTRACT: In recent years, several hacking attacks have broken the security of quantum cryptography implementations by exploiting the presence of losses and the ability of the eavesdropper to tune detection efficiencies. We present a simple attack of this form that applies to any protocol in which the key is constructed from the results of untrusted measurements performed on particles coming from an insecure source or channel. Because of its generality, the attack applies to a large class of protocols, from standard prepareandmeasure to deviceindependent schemes. The derived critical detection efficiencies for security imply that the implementation of most partly device independent solutions is, from the point of view of detection efficiency, almost as demanding as fully deviceindependent ones. We also show how our attack implies the existence of a form of bound randomness, namely nonlocal correlations in which a nonsignalling eavesdropper can find out a posteriori the result of any implemented measurement.  [Show abstract] [Hide abstract]
ABSTRACT: Small quantum absorption refrigerators have recently attracted renewed attention. Here we present a missing design of a twoqubit fridge, the main feature of which is that one of the two machine qubits is itself maintained at a temperature colder than the cold bath. This is achieved by 'reversing' the couplings to the baths compared to previous designs, where only a transition is maintained cold. We characterize the working regime and the efficiency of the fridge. We demonstrate the soundness of the model by deriving and solving a master equation. Finally, we discuss the performance of the fridge, in particular the heat current extracted from the cold bath. We show that our model performs comparably to the standard threelevel quantum fridge, and thus appears appealing for possible implementations of nano thermal machines. 
Article: The most energetic passive states
[Show abstract] [Hide abstract]
ABSTRACT: Passive states are defined as those states that do not allow for work extraction in a cyclic (unitary) process. Within the set of passive states, thermal states are the most stable ones: they maximize the entropy for a given energy, and similarly they minimize the energy for a given entropy. In this article we find the passive states lying in the other extreme, i.e., those that maximize the energy for a given entropy, which we show also minimize the entropy when the energy is fixed. These extremal properties make these states useful to obtain fundamental bounds for the thermodynamics of finite dimensional quantum systems, which we show in several scenarios.  [Show abstract] [Hide abstract]
ABSTRACT: We show how to construct losstolerant linear steering inequalities using a generic set of von Neumann measurements that are violated by $d$dimensional states, and that rely only upon a simple property of the set of measurements used (the maximal overlap between measurement directions). Using these inequalities we show that the critical detection efficiency above which $n$ von Neumann measurements can demonstrate steering is $1/n$. We show furthermore that using our construction and high dimensional states allows for steering demonstrations which are also highly robust to depolarising noise. Finally, our results provide an explicit means to certify the nonjoint measurability of any set of inefficient von Neuman measurements. 
Article: Detecting multipartite entanglement with untrusted measurements in asymmetric quantum networks
[Show abstract] [Hide abstract]
ABSTRACT: The future of quantum communication relies on quantum networks composed by observers sharing multipartite quantum states. The certification of multipartite entanglement will be crucial to the usefulness of these networks. In many real situations it is natural to assume that some observers are more trusted than others in the sense that they have more knowledge of their measurement apparatuses. Here we propose a general method to certify all kinds of multipartite entanglement in this scenario and experimentally demonstrate it in an optical experiment. Our work fills a gap in the characterization of quantum correlations and provides a basis for semideviceindependent cryptographic applications in quantum networks.  [Show abstract] [Hide abstract]
ABSTRACT: We give a simple and intuitive proof that the only states which are completely passive, i.e. those states from which work cannot be extracted even with infinitely many copies, are Gibbs states at positive temperatures. The proof makes use of the idea of virtual temperatures, i.e. the association of temperatures to transitions. We show that (i) passive states are those where every transition is at a positive temperature, and (ii) completely passive states are those where every transition is at the same positive temperature.Physical Review E 12/2014; 91(5). DOI:10.1103/PhysRevE.91.052133 · 2.33 Impact Factor  [Show abstract] [Hide abstract]
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.New Journal of Physics 10/2014; 17(2). DOI:10.1088/13672630/17/2/022003 · 3.67 Impact Factor  [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.Physical Review A 09/2014; 90(6). DOI:10.1103/PhysRevA.90.062109 · 2.99 Impact Factor 
Article: Extracting work from correlations
[Show abstract] [Hide abstract]
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.  [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 06/2014; 5:4185. DOI:10.1038/ncomms5185 · 10.74 Impact Factor  [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. DOI:10.1103/PhysRevLett.112.180404 · 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.New Journal of Physics 04/2014; 17(6). DOI:10.1088/13672630/17/6/065008 · 3.67 Impact Factor  [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. DOI:10.1103/PhysRevE.89.032115 · 2.33 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.New Journal of Physics 01/2014; 16(12). DOI:10.1088/13672630/16/12/123050 · 3.67 Impact Factor 
Article: Extracting work from quantum systems
[Show abstract] [Hide abstract]
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. 
Article: Quantum Cheshire Cats
[Show abstract] [Hide abstract]
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; 15. DOI:10.1088/13672630/15/11/113015 · 3.67 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). DOI:10.1103/PhysRevA.84.022105 · 2.99 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; 85(5). DOI:10.1103/PhysRevE.85.051117 · 2.33 Impact Factor
Publication Stats
340  Citations  
87.08  Total Impact Points  
Top Journals
Institutions

2014–2015

ICFO Institute of Photonic Sciences
Barcino, Catalonia, Spain


2012

University of Cambridge
 Department of Applied Mathematics and Theoretical Physics
Cambridge, England, United Kingdom


2009–2011

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