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Proposed experiment to test the bounds of quantum correlations  [PDF]
Adan Cabello
Physics , 2003, DOI: 10.1103/PhysRevLett.92.060403
Abstract: Clauser-Horne-Shimony-Holt inequality can give values between the classical bound, 2, and Tsirelson's bound, 2 \sqrt 2. However, for a given set of local observables, there are values in this range which no quantum state can attain. We provide the analytical expression for the corresponding bound for a parametrization of the local observables introduced by Filipp and Svozil, and describe how to experimentally trace it using a source of singlet states. Such an experiment will be useful to identify the origin of the experimental errors in Bell's inequality-type experiments and could be modified to detect hypothetical correlations beyond those predicted by quantum mechanics.
Experimental Evidence of Quantum Randomness Incomputability  [PDF]
Cristian S. Calude,Michael J. Dinneen,Monica Dumitrescu,Karl Svozil
Computer Science , 2010, DOI: 10.1103/PhysRevA.82.022102
Abstract: In contrast with software-generated randomness (called pseudo-randomness), quantum randomness is provable incomputable, i.e.\ it is not exactly reproducible by any algorithm. We provide experimental evidence of incomputability --- an asymptotic property --- of quantum randomness by performing finite tests of randomness inspired by algorithmic information theory.
Experimental investigation of classical and quantum correlations under decoherence  [PDF]
Jin-Shi Xu,Xiao-Ye Xu,Chuan-Feng Li,Cheng-Jie Zhang,Xu-Bo Zou,Guang-Can Guo
Physics , 2009, DOI: 10.1038/ncomms1005
Abstract: It is well known that many operations in quantum information processing depend largely on a special kind of quantum correlation, that is, entanglement. However, there are also quantum tasks that display the quantum advantage without entanglement. Distinguishing classical and quantum correlations in quantum systems is therefore of both fundamental and practical importance. In consideration of the unavoidable interaction between correlated systems and the environment, understanding the dynamics of correlations would stimulate great interest. In this study, we investigate the dynamics of different kinds of bipartite correlations in an all-optical experimental setup. The sudden change in behaviour in the decay rates of correlations and their immunity against certain decoherences are shown. Moreover, quantum correlation is observed to be larger than classical correlation, which disproves the early conjecture that classical correlation is always greater than quantum correlation. Our observations may be important for quantum information processing.
Bounds on Quantum Correlations in Bell Inequality Experiments  [PDF]
Yeong-Cherng Liang,Andrew C. Doherty
Physics , 2006, DOI: 10.1103/PhysRevA.75.042103
Abstract: Bell inequality violation is one of the most widely known manifestations of entanglement in quantum mechanics; indicating that experiments on physically separated quantum mechanical systems cannot be given a local realistic description. However, despite the importance of Bell inequalities, it is not known in general how to determine whether a given entangled state will violate a Bell inequality. This is because one can choose to make many different measurements on a quantum system to test any given Bell inequality and the optimization over measurements is a high-dimensional variational problem. In order to better understand this problem we present algorithms that provide, for a given quantum state, both a lower bound and an upper bound on the maximal expectation value of a Bell operator. Both bounds apply techniques from convex optimization and the methodology for creating upper bounds allows them to be systematically improved. In many cases these bounds determine measurements that would demonstrate violation of the Bell inequality or provide a bound that rules out the possibility of a violation. Examples are given to illustrate how these algorithms can be used to conclude definitively if some quantum states violate a given Bell inequality.
Lieb-Robinson bounds and the generation of correlations and topological quantum order  [PDF]
S. Bravyi,M. B. Hastings,F. Verstraete
Physics , 2006, DOI: 10.1103/PhysRevLett.97.050401
Abstract: The Lieb-Robinson bound states that local Hamiltonian evolution in nonrelativistic quantum mechanical theories gives rise to the notion of an effective light-cone with exponentially decaying tails. We discuss several consequences of this result in the context of quantum information theory. First, we show that the information that leaks out to space-like separated regions is negligable, and that there is a finite speed at which correlations and entanglement can be distributed. Second, we discuss how these ideas can be used to prove lower bounds on the time it takes to convert states without topological quantum order to states with that property. Finally, we show that the rate at which entropy can be created in a block of spins scales like the boundary of that block.
Experimental Evidence for Quantum Structure in Cognition  [PDF]
Diederik Aerts,Sven Aerts,Liane Gabora
Physics , 2008, DOI: 10.1007/978-3-642-00834-4_7
Abstract: We proof a theorem that shows that a collection of experimental data of membership weights of items with respect to a pair of concepts and its conjunction cannot be modeled within a classical measure theoretic weight structure in case the experimental data contain the effect called overextension. Since the effect of overextension, analogue to the well-known guppy effect for concept combinations, is abundant in all experiments testing weights of items with respect to pairs of concepts and their conjunctions, our theorem constitutes a no-go theorem for classical measure structure for common data of membership weights of items with respect to concepts and their combinations. We put forward a simple geometric criterion that reveals the non classicality of the membership weight structure and use experimentally measured membership weights estimated by subjects in experiments to illustrate our geometrical criterion. The violation of the classical weight structure is similar to the violation of the well-known Bell inequalities studied in quantum mechanics, and hence suggests that the quantum formalism and hence the modeling by quantum membership weights can accomplish what classical membership weights cannot do.
Experimental Evidence for Efimov Quantum States  [PDF]
H. -C. Naegerl,T. Kraemer,M. Mark,P. Waldburger,J. G. Danzl,B. Engeser,A. D. Lange,K. Pilch,A. Jaakkola,C. Chin,R. Grimm
Physics , 2006, DOI: 10.1063/1.2400657
Abstract: Three interacting particles form a system which is well known for its complex physical behavior. A landmark theoretical result in few-body quantum physics is Efimov's prediction of a universal set of weakly bound trimer states appearing for three identical bosons with a resonant two-body interaction. Surprisingly, these states even exist in the absence of a corresponding two-body bound state and their precise nature is largely independent of the particular type of the two-body interaction potential. Efimov's scenario has attracted great interest in many areas of physics; an experimental test however has not been achieved. We report the observation of an Efimov resonance in an ultracold thermal gas of cesium atoms. The resonance occurs in the range of large negative two-body scattering lengths and arises from the coupling of three free atoms to an Efimov trimer. We observe its signature as a giant three-body recombination loss when the strength of the two-body interaction is varied near a Feshbach resonance. This resonance develops into a continuum resonance at non-zero collision energies, and we observe a shift of the resonance position as a function of temperature. We also report on a minimum in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point from which to explore the universal properties of resonantly interacting few-body systems.
Quantum correlations versus Multisimultaneity: an experimental test  [PDF]
Andre Stefanov,Nicolas Gisin,Antoine Suarez,Hugo Zbinden
Physics , 2001, DOI: 10.1103/PhysRevLett.88.120404
Abstract: Multisimultaneity is a causal model of relativistic quantum physics which assigns a real time ordering to any set of events, much in the spirit of the pilot-wave picture. Contrary to standard quantum mechanics, it predicts a disappearance of the correlations in a Bell-type experiment when both analysers are in relative motion such that, each one in its own inertial reference frame, is first to select the output of the photons. We tested this prediction using acousto-optic modulators as moving beam-splitters and interferometers separated by 55 m. We didn't observe any disappearance of the correlations, thus refuting Multisimultaneity.
Remarks on experimental bounds on quantum gravity effects on fermions  [PDF]
Gustavo Dotti,Reinaldo J. Gleiser
Physics , 2003, DOI: 10.1103/PhysRevD.67.101501
Abstract: Effects of space time geometry fluctuations on fermionic fields have recently been looked for in nuclear physics experiments, and were found to be much lower than predicted, at a phenomenological level, by loop quantum gravity. We show that possible corrections to the canonical structure in the semi classical regime may introduce important changes in the outcome of the theory, and may explain the observed mismatch with experiments.
Experimental demonstration of quantum correlations over more than 10 km  [PDF]
W. Tittel,J. Brendel,B. Gisin,T. Herzog,H. Zbinden,N. Gisin
Physics , 1997, DOI: 10.1103/PhysRevA.57.3229
Abstract: Energy and time entangled photons at a wavelength of 1310 nm are produced by parametric downconversion in a KNbO3 crystal and are sent into all-fiber interferometers using a telecom fiber network. The two interferometers of this Franson-type test of the Bell-inequality are located 10.9 km apart from one another. Two-photon fringe visibilities of up to 81.6 % are obtained. These strong nonlocal correlations support the nonlocal predictions of quantum mechanics and provide evidence that entanglement between photons can be maintained over long distances.
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