Publications (192)723.69 Total impact
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ABSTRACT: Contextuality provides a unifying paradigm for nonclassical aspects of quantum probabilities and resources of quantum information. Unfortunately, most forms of quantum contextuality remain experimentally unexplored due to the difficulty of performing sequences of projective measurements on individual quantum systems. Here we show that twopoint correlations between dichotomic compatible observables are sufficient to reveal any form of contextuality. This allows us to design simple experiments that are more robust against imperfections and easier to analyze, thus opening the door for observing interesting forms of contextuality, including those requiring quantum systems of high dimensions. In addition, this result allows us to connect contextuality to communication complexity scenarios and reformulate a recent result relating contextuality and quantum computation.  [Show abstract] [Hide abstract]
ABSTRACT: A unifying principle explaining the numerical bounds of quantum correlations remains elusive despite the efforts devoted to identify it. Here we show that these bounds are indeed not exclusive to quantum theory: for any abstract correlation scenario with compatible measurements, a suited classical wave theory produces probability distributions indistinguishable from those of quantum theory, and therefore share the same bounds. We demonstrate this finding by implementing classical microwaves propagating along metersize transmissionline circuits and reproduce the probabilities of three emblematic quantum experiments. Our results show that what distinguishes quantum theory is not the set of numerical bounds, but the fact that it is produced without the resources used by classical systems. The implications of this observation are discussed.  [Show abstract] [Hide abstract]
ABSTRACT: Measurement scenarios containing events with relations of exclusivity represented by pentagons, heptagons, nonagons, etc., or their complements are the only ones in which quantum probabilities cannot be described classically. Interestingly, quantum theory predicts that the maximum values for any of these graphs cannot be achieved in Bell inequality scenarios. With the exception of the pentagon, this prediction remained experimentally unexplored. Here we test the quantum maxima for the heptagon and the complement of the heptagon using three and fivedimensional quantum states, respectively. In both cases, we adopt two different encodings: linear transverse momentum and orbital angular momentum of single photons. Our results exclude maximally noncontextual hiddenvariable theories and are in good agreement with the maxima predicted by quantum theory.  [Show abstract] [Hide abstract]
ABSTRACT: The interpretation of quantum theory is one of the longeststanding debates in physics. TypeI interpretations see quantum probabilities as determined by intrinsic properties of the world. TypeII interpretations see quantum probabilities as not directly dealing with intrinsic properties of the world but with relational experiences between an observer and the world. It is usually believed that deciding between these two types cannot be made simply on purely physical grounds but it requires an act of metaphysical judgement. Here we show that, although the problem is undecidable within the framework of quantum theory, it is decidable, under some assumptions, within the framework of thermodynamics. We prove that typeI interpretations are incompatible with the following assumptions: (i) the decision of which measurement is performed on a quantum system can be made independently of the system, (ii) a quantum system has limited memory, and (iii) Landauer's principle is valid. We consider an ideal experiment in which an individual quantum system is submitted to a sequence of quantum projective measurements that leave the system in pure quantum states. We show that in any typeI interpretation satisfying (i)(iii) the system must reset its internal state, which implies that a minimum amount of heat per measurement has to be dissipated into the system's environment. We calculate a lower bound to the heat dissipated per measurement assuming that the measurements are chosen from a set of size $2^n$. Then, we show that this lower bound becomes infinite in the limit of $n$ tending to infinity. This leads to the conclusion that either typeI interpretations are untenable or at least one of the assumptions (i)(iii) has to be abandoned.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum nonlocality can be revealed "via local contextuality" in quditqudit entangled systems with $d > 2$, that is, through the violation of inequalities containing only AliceBob correlations that admit a local description, and AliceAlice correlations (between the results of sequences of measurements on Alice's subsystem) that admit a local (but contextual) description. A fundamental question to understand the respective roles of entanglement and local contextuality is whether nonlocality via local contextuality exists when the parties have only qubitqubit entanglement. Here we respond affirmatively to this question. This result further clarifies the connection between contextuality and nonlocality and opens the door for observing nonlocality via local contextuality in actual experiments.  [Show abstract] [Hide abstract]
ABSTRACT: Deviceindependent quantum communication will require a loopholefree violation of Bell inequalities. In typical scenarios where line of sight between the communicating parties is not available, it is convenient to use energytime entangled photons due to intrinsic robustness while propagating over optical fibers. Here we show an energytime ClauserHorneShimonyHolt Bell inequality violation with two parties separated by 3.7 km over the deployed optical fiber network belonging to the University of Concepción in Chile. Remarkably, this is the first Bell violation with spatially separated parties that is free of the postselection loophole, which affected all previous infield longdistance energytime experiments. Our work takes a further step towards a fiberbased loopholefree Bell test, which is highly desired for secure quantum communication due to the widespread existing telecommunication infrastructure.  [Show abstract] [Hide abstract]
ABSTRACT: Contextuality is a fundamental feature of quantum theory and a necessary resource for quantum computation and communication. It is therefore important to investigate how large can contextuality be in quantum theory. Contextuality witnesses can be expressed as a sum $S$ of $n$ probabilities, such that $1 \le \alpha < \vartheta \le n$ are, respectively, the maximum of $S$ for noncontextual theories and for the theory under consideration. A theory allows for absolute maximal contextuality if it has scenarios in which $\vartheta/\alpha$ tends to $n$. Here we show that quantum theory allows for absolute maximal contextuality despite what is suggested by the examination of the quantum violations of Bell and noncontextuality inequalities considered in the past. Our proof is not constructive and does not single out explicit scenarios. Nevertheless, we identify scenarios in which quantum theory allows for almost absolute maximal contextuality.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum theory introduces a cut between the observer and the observed system, but does not provide a definition of what is an observer. Based on an informational definition of observer, Grinbaum has recently predicted an upper bound on bipartite correlations in the ClauserHorneShimonyHolt (CHSH) Bell scenario equal to 2.82537, which is slightly smaller than the Tsirelson bound of standard quantum theory, but is consistent with all the available experimental results. Not being able to exceed Grinbaum's limit would support that quantum theory is only an effective description of a more fundamental theory and would have a deep impact in physics and quantum information processing. Here we present a test of the CHSH Bell inequality on photon pairs in maximally entangled states of polarization in which a value 2.8276+0.00082 is observed, violating Grinbaum's bound by 2.72 standard deviations and providing the smallest distance with respect to Tsirelson's bound ever reported, namely, 0.0008+0.00082. This sets a new lower experimental bound for Tsirelson's bound, strengthening the value of principles that predict Tsirelson's bound as possible explanations of all natural limits of correlations, and has important consequences for cryptographic security, randomness certification, characterization of physical properties in deviceindependent scenarios, and certification of quantum computation.  [Show abstract] [Hide abstract]
ABSTRACT: There is a tension in quantum theory between the existence of a widely accepted way to axiomatize the theory and the lack of similarly accepted intuitive principles from which the theory can be derived. This tension is present at the very definition of what measurements are admissible. The usual assumption is that all measurements which do not produce negative probabilities are valid measurements. However, besides simplicity, there is no conceptional reason for such an assumption. Here we show that this assumption leads to a very particular prediction that can be experimentally tested: in certain situations, the number of outcomes of a measurement is, by itself, a quantum phenomenon. For reaching this conclusion, we consider minor modifications of quantum theory in which all measurements are produced from quantum measurements with a limited number of outcomes and classical measurements, and show that any of these modifications is accessible to falsification by particular highprecision Belltype experiments. Our analysis reveals that the results of previous experiments provide evidence that nature cannot be explained with the simplest of these modifications in which all measurements are essentially quantum dichotomic. This supports standard quantum theory versus a natural and almost indistinguishable alternative, but leaves as an open challenge to perform experiments which allow us to exclude other alternatives such as essentially trichotomic quantum theories and more general dichotomic theories.  [Show abstract] [Hide abstract]
ABSTRACT: Deviceindependent (DI) quantum communication will require a loopholefree violation of Bell inequalities. In typical scenarios where lineofsight between the communicating parties is not available, it is convenient to use energytime entangled photons due to intrinsic robustness while propagating over optical fibers. Here we show an energytime ClauserHorneShimonyHolt Bell inequality violation with two parties separated by 3.7 km over the deployed optical fiber network belonging to the University of Concepci\'on in Chile. Remarkably, this violation is free of the postselection loophole affecting all previous infield longdistance energytime experiments. Our work takes a further step towards a fiberbased loopholefree Bell test, which is highly desired for secure quantum communication due to the widespread existing telecommunication infrastructure.  [Show abstract] [Hide abstract]
ABSTRACT: Clock synchronization for nonfaulty processes in multiprocess networks is indispensable for a variety of technologies. A reliable system must be able to resynchronize the nonfaulty processes upon some components failing causing the distribution of incorrect or conflicting information in the network. The task of synchronizing such networks is related to Byzantine agreement (BA), which can classically be solved using recursive algorithms if and only if less than onethird of the processes are faulty. Here we introduce a nonrecursive quantum algorithm, based on a quantum solution of the detectable BA, which achieves clock synchronization in the presence of arbitrary many faulty processes by using only a single quantum system.  [Show abstract] [Hide abstract]
ABSTRACT: We solve the problem of whether a set of quantum tests reveals stateindependent contextuality and use this result to identify the simplest set of minimal dimension. We also show that identifying stateindependent contextuality graphs [R. Ramanathan and P. Horodecki, Phys. Rev. Lett. 112, 040404 (2014)] is not sufficient for revealing stateindependent contextuality.  [Show abstract] [Hide abstract]
ABSTRACT: Contextuality is a fundamental property of quantum theory and a critical resource for quantum computation. Here, we experimentally observe the arguably cleanest form of contextuality in quantum theory [A. Cabello et al., Phys. Rev. Lett. 111, 180404 (2013)] by implementing a novel method for performing two sequential measurements on heralded photons. This method opens the door to a variety of fundamental experiments and applications.  [Show abstract] [Hide abstract]
ABSTRACT: We show that, for general probabilistic theories admitting sharp measurements, the exclusivity principle together with two assumptions exactly singles out the Tsirelson bound of the ClauserHorneShimonyHolt Bell inequality.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum $n$body correlations violate inequalities for nonlocal theories in which arbitrary correlations are allowed within any strict subset of bodies while only local correlations are allowed between the subsets. Violations of these inequalities have been recently observed up to $n=6$. An important question is why the universe is exactly as genuinely $n$body nonlocal as predicted by quantum theory, but not more or less. Here we prove that, for any $n>2$, the exclusivity principle and two assumptions (namely, independence of remote experiments and existence of a joint probability distribution for certain measurements), give, for general probabilistic theories, the same genuinely $n$body nonlocality found in quantum theory.  [Show abstract] [Hide abstract]
ABSTRACT: Random numbers are essential for a wide range of applications, including cryptography, financial security, and digital rights management. However, producing random numbers from a finite state machine, such as a classical computer, is impossible. One alternative is to use quantum random number generators (QRNG), which explore unpredictable results of quantum phenomena to extract a string of random bits. Unfortunately, however, commercial QRNGs have also to rely on assumptions on the internal workings of its devices. "Deviceindependent" QRNGs are nevertheless possible, but are impractical since they require Bellinequality violations free of the detection loophole. Here, we introduce a new protocol for quantum randomness extraction which works even in the case of very low detection efficiency, and where no internal device description is needed. The method is based on a new tensorlike indicator of randomness, and there is only one assumption: that the dimension of the preparedandmeasured quantum system is upper bounded, as in the semideviceindependent (SDI) paradigm. We implement the protocol using weak coherent states and standard singlephoton detectors. Our results pave the way towards a second generation of practical and more secure QRNGs.  [Show abstract] [Hide abstract]
ABSTRACT: KochenSpecker (KS) sets are key tools for proving some fundamental results in quantum theory and also have potential applications in quantum information processing. However, so far, their intrinsic complexity has prevented experimentalists from using them for any application. The KS set requiring the smallest number of contexts has been recently found. Relying on this simple KS set, here we report an input stateindependent experimental technique to certify whether a set of measurements is actually accessing a preestablished quantum sixdimensional space encoded in the transverse momentum of single photons.  [Show abstract] [Hide abstract]
ABSTRACT: For eightdimensional quantum systems there is a KochenSpecker (KS) set of 40 quantum yesno tests that is related to the GreenbergerHorneZeilinger (GHZ) proof of Bell's theorem. Here we experimentally implement this KS set using an eightdimensional Hilbert space spanned by the transverse momentum of single photons. We show that the experimental results of these tests violate a stateindependent noncontextuality inequality. In addition, we show that, if the system is prepared in states that are formally equivalent to a threequbit GHZ and W states, then the results of a subset of 16 tests violate a noncontextuality inequality that is formally equivalent to the threeparty Mermin's Bell inequality, but for single eightdimensional quantum systems. These experimental results highlight the connection between quantum contextuality and nonlocality for eightdimensional quantum systems.  [Show abstract] [Hide abstract]
ABSTRACT: Noncontextuality (NC) and Bell inequalities can be expressed as bounds $\Omega$ for positive linear combinations $S$ of probabilities of events, $S \leq \Omega$. Exclusive events in $S$ can be represented as adjacent vertices of a graph called the exclusivity graph of $S$. In the case that events correspond to the outcomes of quantum projective measurements, quantum probabilities are intimately related to the Gr\"otschelLov\'aszSchrijver theta body of the exclusivity graph. Then, one can easily compute an upper bound to the maximum quantum violation of any NC or Bell inequality by optimizing $S$ over the theta body and calculating the Lov\'asz number of the corresponding exclusivity graph. In some cases, this upper bound is tight and gives the exact maximum quantum violation. However, in general, this is not the case. The reason is that the exclusivity graph does not distinguish among the different ways exclusivity can occur in Bellinequality (and similar) scenarios. An interesting question is whether there is a graphtheoretical concept which accounts for this problem. Here we show that, for any given $N$partite Bell inequality, an edgecoloured multigraph composed on $N$ singlecolour graphs can be used to encode the relationships of exclusivity between each party's parts of the events. Then, the maximum quantum violation of the Bell inequality is exactly given by a refinement of the Lov\'asz number that applies to these edgecoloured multigraphs. We show how to calculate upper bounds for this number using a hierarchy of semidefinite programs and calculate upper bounds for $I_3$, $I_{3322}$ and the three bipartite Bell inequalities whose exclusivity graph is a pentagon. The multigraphtheoretical approach introduced here may remove some obstacles in the program of explaining quantum correlations from first principles.  [Show abstract] [Hide abstract]
ABSTRACT: We show that the exclusivity principle exactly singles out the Tsirelson bound of the ClauserHorneShimonyHolt Bell inequality. The proof is surprisingly simple and does not require an infinite universe.
Publication Stats
4k  Citations  
723.69  Total Impact Points  
Top Journals
 Physical Review A (59)
 Physical Review Letters (32)
 Physical Review Letters (16)
 Physics Letters A (11)
 Foundations of Physics (4)
Institutions

19972015

Universidad de Sevilla
 Applied Physics III
Hispalis, Andalusia, Spain


20112013

Stockholm University
 Department of Physics
Tukholma, Stockholm, Sweden


2010

University of Innsbruck
 Department of Theoretical Physics
Innsbruck, Tyrol, Austria


19971998

Complutense University of Madrid
 Department of Theoretical physics I
Madrid, Madrid, Spain
