Publications (175)700.83 Total impact

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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.Scientific Reports 01/2015; 5:7982. DOI:10.1038/srep07982 · 5.08 Impact Factor 
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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. 
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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.Physical Review Letters 12/2014; 113(25):250403. DOI:10.1103/PhysRevLett.113.250403 · 7.73 Impact Factor 
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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. 
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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(9):090404. DOI:10.1103/PhysRevLett.113.090404 · 7.73 Impact Factor 
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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(1):012119. DOI:10.1103/PhysRevA.90.012119 · 2.99 Impact Factor 
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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.Journal of Physics A Mathematical and Theoretical 07/2014; 47(42). DOI:10.1088/17518113/47/42/424021 · 1.77 Impact Factor 
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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. 
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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. 
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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. DOI:10.1103/PhysRevLett.112.140503 · 7.73 Impact Factor 
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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. DOI:10.1103/PhysRevLett.112.100401 · 7.73 Impact Factor 
Conference Paper: Experimental relativistic quantum bit commitment
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ABSTRACT: 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 provides an experimental proof of unconditional secure bit commitment protocol and demonstrates the experimental feasibility of relativistic quantum communication.APS March Meeting; 03/2014 
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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.Physical Review A 02/2014; 89(3). DOI:10.1103/PhysRevA.89.030101 · 2.99 Impact Factor 
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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. DOI:10.1103/PhysRevLett.112.040401 · 7.73 Impact Factor 
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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. DOI:10.1103/PhysRevLett.112.010504 · 7.73 Impact Factor 
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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.Physical Review A 12/2013; 89(1). DOI:10.1103/PhysRevA.89.012102 · 2.99 Impact Factor 
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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. DOI:10.1038/ncomms3871 · 10.74 Impact Factor 
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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. 
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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. DOI:10.1103/PhysRevLett.111.180404 · 7.73 Impact Factor 
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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.
Publication Stats
3k  Citations  
700.83  Total Impact Points  
Top Journals
 Physical Review A (56)
 Physical Review Letters (30)
 Physical Review Letters (14)
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 Foundations of Physics (4)
Institutions

1997–2015

Universidad de Sevilla
 Applied Physics III
Hispalis, Andalusia, Spain


2013

Nankai University
 Department of Physics
T’ienchingshih, Tianjin Shi, China


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


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


2006–2007

Universität Heidelberg
 Institute of Physical Chemistry
Heidelburg, BadenWürttemberg, Germany


2005

National University of Singapore
 Department of Physics
Tumasik, Singapore


1997–2005

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