Publications (162)612.15 Total impact
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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.Physical Review Letters 08/2014; 113:090404. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Contextuality is a fundamental concept for understanding quantum probabilities and the origin of the power of quantum systems for computation and information processing. A natural question is: Which is the simplest and cleanest version of quantum contextuality? It has been very recently found that there is a version that is analog to "the simplest and cleanest" version of quantum nonlocality, but only requires single threelevel quantum systems (qutrits) rather than composite systems. Here we report the first experimental observation of this "Hardylike" quantum contextuality. We measured the correlations between the outcomes of five combinations of two compatible measurements performed sequentially, in any possible order, on heralded photonic pathencoded qutrits. The experiment adopts a novel configuration which allows for performing two sequential measurements on the same photon with a high fidelity and allows for observing the independence of the correlations with respect to the order in which measurements are performed. The experimental results match the conditions needed to define a Hardylike argument and the predictions of quantum theory. In addition, they violate the relevant noncontextuality inequality, even when the noncontextual bound is corrected to take into account experimental imperfections. The experimental observation of this form of quantum contextuality is of fundamental importance, since it connects the KochenSpecker theorem with the simplest noncontextuality inequality. In addition, the method introduced here opens the door to applications requiring sequential quantum measurements on photonic systems.08/2014;  [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.Physical Review A 07/2014; 90:012119. · 3.04 Impact Factor  [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.07/2014;  [Show abstract] [Hide abstract]
ABSTRACT: Several studies in recent years have demonstrated that upperdivision students struggle with the mathematics of thermodynamics. This paper presents a task analysis based on several expert attempts to solve a challenging mathematics problem in thermodynamics. The purpose of this paper is twofold. First, we highlight the importance of cognitive task analysis for understanding expert performance and show how the epistemic games framework can be used as a tool for this type of analysis, with thermodynamics as an example. Second, through this analysis, we identify several issues related to thermodynamics that are relevant to future research into student understanding and learning of the mathematics of thermodynamics.05/2014; 10.  [Show abstract] [Hide abstract]
ABSTRACT: An important problem in quantum information processing is the certification of the dimension of quantum systems without making assumptions about the devices used to prepare and measure them, that is, in a deviceindependent manner. A crucial question is whether such certification is experimentally feasible for highdimensional quantum systems. Here we experimentally witness in a deviceindependent manner the generation of sixdimensional quantum systems encoded in the orbital angular momentum of single photons and show that the same method can be scaled, at least, up to dimension 13.Physical Review Letters 04/2014; 112(14):140503. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that the nodisturbance principle imposes a tradeoff between locally contextual correlations violating the KlyachkoCanBiniciogˇluShumovski inequality and spatially separated correlations violating the ClauserHorneShimonyHolt inequality. The violation of one inequality forbids the violation of the other. We also obtain the corresponding monogamy relation imposed by quantum theory for a qutritqubit system. Our results show the existence of fundamental monogamy relations between contextuality and nonlocality that suggest that entanglement might be a particular form of a more fundamental resource.Physical Review Letters 03/2014; 112(10):100401. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that the exclusivity (E) principle singles out the set of quantum correlations associated with any exclusivity graph assuming the set of quantum correlations for the complementary graph. Moreover, we prove that, for selfcomplementary graphs, the E principle, by itself (i.e., without further assumptions), excludes any set of correlations strictly larger than the quantum set. Finally, we prove that, for vertextransitive graphs, the E principle singles out the maximum value for the quantum correlations assuming only the quantum maximum for the complementary graph. This opens the door for testing the impossibility of higherthanquantum correlations in experiments.02/2014; 89(3).  [Show abstract] [Hide abstract]
ABSTRACT: Correlations in Bell and noncontextuality inequalities can be expressed as a positive linear combination of probabilities of events. Exclusive events can be represented as adjacent vertices of a graph, so correlations can be associated to a subgraph. We show that the maximum value of the correlations for classical, quantum, and more general theories is the independence number, the Lovász number, and the fractional packing number of this subgraph, respectively. We also show that, for any graph, there is always a correlation experiment such that the set of quantum probabilities is exactly the GrötschelLovászSchrijver theta body. This identifies these combinatorial notions as fundamental physical objects and provides a method for singling out experiments with quantum correlations on demand.Physical Review Letters 01/2014; 112(4):040401. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Quantum physics allows for unconditionally secure communication between parties that trust each other. However, when the parties do not trust each other such as in the bit commitment scenario, quantum physics is not enough to guarantee security unless extra assumptions are made. Unconditionally secure bit commitment only becomes feasible when quantum physics is combined with relativistic causality constraints. Here we experimentally implement a quantum bit commitment protocol with relativistic constraints that offers unconditional security. The commitment is made through quantum measurements in two quantum key distribution systems in which the results are transmitted via freespace optical communication to two agents separated with more than 20 km. The security of the protocol relies on the properties of quantum information and relativity theory. In each run of the experiment, a bit is successfully committed with less than 5.68×10^{2} cheating probability. This demonstrates the experimental feasibility of quantum communication with relativistic constraints.Physical Review Letters 01/2014; 112(1):010504. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that for twoqubit chained Bell inequalities with an arbitrary number of measurement settings, nonlocality and entanglement are not only different properties but are inversely related. Specifically, we analytically prove that in absence of noise, robustness of nonlocality, defined as the maximum fraction of detection events that can be lost such that the remaining ones still do not admit a local model, and concurrence are inversely related for any chained Bell inequality with an arbitrary number of settings. The closer quantum states are to product states, the harder it is to reproduce quantum correlations with local models. We also show that, in presence of noise, nonlocality and entanglement are simultaneously maximized only when the noise level is equal to the maximum level tolerated by the inequality; in any other case, a more nonlocal state is always obtained by reducing the entanglement. In addition, we observed that robustness of nonlocality and concurrence are also inversely related for the Bell scenarios defined by the tight twoqubit threesetting I3322 inequality, and the tight twoqutrit inequality I3.12/2013; 89(1).  [Show abstract] [Hide abstract]
ABSTRACT: Any practical realization of entanglementbased quantum communication must be intrinsically secure and able to span long distances avoiding the need of a straight line between the communicating parties. The violation of Bell's inequality offers a method for the certification of quantum links without knowing the inner workings of the devices. Energytime entanglement quantum communication satisfies all these requirements. However, currently there is a fundamental obstacle with the standard configuration adopted: an intrinsic geometrical loophole that can be exploited to break the security of the communication, in addition to other loopholes. Here we show the first experimental Bell violation with energytime entanglement distributed over 1 km of optical fibres that is free of this geometrical loophole. This is achieved by adopting a new experimental design, and by using an actively stabilized fibrebased long interferometer. Our results represent an important step towards longdistance secure quantum communication in optical fibres.Nature Communications 11/2013; 4:2871. · 10.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Why do correlations between the results of measurements performed on physical systems violate Bell and noncontextuality inequalities up to some specific limits? The answer may follow from the observation that in quantum theory, unlike in other theories, whenever there is an experiment to measure $A$ simultaneously with $B$, another to measure $B$ with $C$, and another to measure $A$ with $C$, there is always an experiment to measure all of them simultaneously. This property implies that quantum theory satisfies a seemingly irrelevant restriction called the exclusivity (E) principle which, surprisingly, explains the set of quantum correlations in some fundamental scenarios. An open problem is whether the E principle explains the maximum quantum violation of the BellCHSH inequality. Here we show experimentally that the E principle imposes an upper bound to the violation of the BellCHSH inequality that matches the maximum predicted by quantum theory. For that, we use the result of an independent experiment testing a specific noncontextuality inequality. We perform both experiments: the BellCHSH inequality experiment on polarization entangled states of pairs of photons in Stockholm and, to demonstrate independence, the noncontextuality inequality experiment on single photons' orbital angular momentum states in Rome. The observed results provide the first experimental evidence that the E principle determines the limits of quantum correlations and prove that hypothetical superquantum violations for either experiment would violate the E principle. This supports the conclusion that the E principle captures a fundamental limitation of nature. If this is true, much of quantum theory trivially follow from merely taking the E principle to be a fundamental truth, and various informationtheoretic postulates are also simplified and/or strengthened.11/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Contextuality and nonlocality are two fundamental properties of nature. Hardy's proof is considered the simplest proof of nonlocality and can also be seen as a particular violation of the simplest Bell inequality. A fundamental question is: Which is the simplest proof of contextuality? We show that there is a Hardylike proof of contextuality that can also be seen as a particular violation of the simplest noncontextuality inequality. Interestingly, this new proof connects this inequality with the proof of the KochenSpecker theorem, providing the missing link between these two fundamental results, and can be extended to an arbitrary odd number n of settings, an extension that can be seen as a particular violation of the ncycle inequality.Physical Review Letters 11/2013; 111(18):180404. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: There are two fundamental theorems of impossibility of hidden variables in quantum theory: the KochenSpecker (KS) theorem, which excludes noncontextual hidden variables for any quantum state, and Bell's theorem that rules out local hidden variables for quantum entangled states. In this work we experimentally observe that, for 8dimensional quantum systems, both theorems unify in a simple proof. First, we implement a KS set of 40 quantum yesno tests on 8dimensional quantum systems encoded in the transverse momentum of single photons, and observe the stateindependent quantum violation of an inequality satisfied by noncontextual hidden variables. Then, we show that, if the result of one of the KS tests is positive, the results of 16 of the remaining KS tests violate a Bell inequality satisfied by local hidden variables. Our work highlights the fundamental connection between quantum contextuality and nonlocality.08/2013; 
Article: The simplest KochenSpecker set
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ABSTRACT: KochenSpecker (KS) theorem is a fundamental result that shows that quantum theory cannot be explained if measurement results were predefined. Its proof requires a set of yesno questions each of them in several contexts. The original KS proof required 132 contexts and, so far, the simplest known proof requires 9 contexts. Here we present a proof with only 7 contexts and show that, under some assumptions, this is the smallest number of contexts possible in a KS proof. Finally, we use our 7context proof to derive an experimentally testable stateindependent noncontextuality inequality.08/2013;  [Show abstract] [Hide abstract]
ABSTRACT: Hardy's proof is considered the simplest proof of nonlocality. Here we introduce an equally simple proof that (i) has Hardy's as a particular case, (ii) shows that the probability of nonlocal events grows with the dimension of the local systems, and (iii) is always equivalent to the violation of a tight Bell inequality.08/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We report two fundamental experiments on threelevel quantum systems (qutrits). The first one tests the simplest task for which quantum mechanics provides an advantage with respect to classical physics. The quantum advantage is certified by the violation of Wright's inequality, the simplest classical inequality violated by quantum mechanics. In the second experiment, we obtain contextual correlations by sequentially measuring pairs of compatible observables on a qutrit, and show the violation of Klyachko et al.'s inequality, the most fundamental noncontextuality inequality violated by qutrits. Our experiment tests exactly Klyachko et al.'s inequality, uses the same measurement procedure for each observable in every context, and implements the sequential measurements in any possible order.Scientific Reports 07/2013; 3:2170. · 5.08 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: EinsteinPodolskyRosen steering is a form of quantum nonlocality intermediate between entanglement and Bell nonlocality. Although Schrödinger already mooted the idea in 1935, steering still defies a complete understanding. In analogy to "allversusnothing" proofs of Bell nonlocality, here we present a proof of steering without inequalities rendering the detection of correlations leading to a violation of steering inequalities unnecessary. We show that, given any twoqubit entangled state, the existence of certain projective measurement by Alice so that Bob's normalized conditional states can be regarded as two different pure states provides a criterion for AlicetoBob steerability. A steering inequality equivalent to the allversusnothing proof is also obtained. Our result clearly demonstrates that there exist many quantum states which do not violate any previously known steering inequality but are indeed steerable. Our method offers advantages over the existing methods for experimentally testing steerability, and sheds new light on the asymmetric steering problem.Scientific Reports 07/2013; 3:2143. · 5.08 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a protocol for everlasting unconditionally secure bit commitment based on quantum nonlocality. This protocol recovers two features missing in recent relativistic bit commitment protocols, namely, that the committer can decide if and when she wants to reveal the commitment and that the security of the commitment lasts for arbitrary long time. The protocol shows that quantum nonlocality is a key ingredient for solving another fundamental cryptographic problem.06/2013;
Publication Stats
2k  Citations  
612.15  Total Impact Points  
Top Journals
 Physical Review A (51)
 Physical Review Letters (29)
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Institutions

1997–2014

Universidad de Sevilla
 Departamento de Física Aplicada III
Hispalis, Andalusia, Spain


2011–2013

Stockholm University
 Department of Physics
Stockholm, Stockholm, Sweden


2012

Universität Siegen
 Faculty IV: Science and Technology
Siegen, North RhineWestphalia, Germany 
Federal University of Minas Gerais
 Departamento de Física
Cidade de Minas, Minas Gerais, Brazil


2009–2012

Sapienza University of Rome
 Department of Physics
Roma, Latium, Italy


2010

University of Science and Technology of China
 Department of Modern Physics
Hefei, Anhui Sheng, China


2008–2010

University of Innsbruck
 Department of Theoretical Physics
Innsbruck, Tyrol, Austria 
LudwigMaximilianUniversity of Munich
 Faculty of Physics
München, Bavaria, Germany


2004–2008

Max Planck Institute of Quantum Optics
Arching, Bavaria, Germany


2006

Consorzio Nazionale Interuniversitario per le Scienze del Mare
Roma, Latium, Italy 
Universität Heidelberg
 Institute of Physical Chemistry
Heidelberg, BadenWuerttemberg, Germany


1997–1998

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