976 publications from this institution
John Bell proposed an ontology for the GRW (Ghirardi, Rimini, and Weber) modification of quantum mechanics in terms of flashes occurring at space-time points. This article spells out the motivation for this ontology, inquires into the status of the wave function in it, critically examines the claim of its being Lorentz invariant, and considers whether it is a parsimonious but nevertheless physically adequate ontology.
The slogan information is physical has been so successful that it led to some excess. Classical and quantum information can be thought of independently of any physical implementation. Pure information tasks can be realized using such abstract c- and qu-bits, but physical tasks require appropriate physical realizations of c- or qu-bits. As illustration we consider the problem of communicating chirality.
We present new inequalities for testing Leggett's model [Found. Phys. 33, 1469 (2003)] of non-local quantum correlations. Remarkably these inequalities can be derived in a very simple way, assuming only the non-negativity of probability distributions. The simplest of these inequalities is experimentally violated. We also investigate more general models a la Leggett, and show that such models must have vanishing marginals in order to reproduce the correlations of the singlet state.
We present an algebraic description of the sets of local correlations in arbitrary networks, when the parties have finite inputs and outputs. We consider networks generalizing the usual Bell scenarios by the presence of multiple uncorrelated sources. We prove a finite upper bound on the cardinality of the value sets of the local hidden variables. Consequently, we find that the sets of local correlations are connected, closed and semialgebraic, and bounded by tight polynomial Bell-like inequalities.
It has been shown theoretically that a light amplifier working on the physical principle of stimulated emission should achieve optimal quantum cloning of the polarization state of light. We demonstrate close-to-optimal universal quantum cloning of polarization in a standard fiber amplifier for telecom wavelengths. For cloning 1-->2 we find a fidelity of 0.82, the optimal value being 5 / 6=0.83.
Quantum cryptography could be the first application of quantum mechanics at the individual quanta level. Results using a commercial prototype over 67 km of telecom cables will be presented. The dialog between fundamental and applied issues will be illustrated by the related demonstrations of quantum nonlocality and of quantum teleportation.
We consider the Bennett-Brassard cryptographic scheme, which uses two conjugate quantum bases. An eavesdropper who attempts to obtain information on qubits sent in one of the bases causes a disturbance to qubits sent in the other basis. We derive an upper bound to the accessible information in one basis, for a given error rate in the conjugate basis. Independently fixing the error rates in the conjugate bases, we show that both bounds can be attained simultaneously by an optimal eavesdropping probe. The probe interaction and its subsequent measurement are described explicitly. These results are combined to give an expression for the optimal information an eavesdropper can obtain for a given average disturbance when her interaction and measurements are performed signal by signal. Finally, the relation between quantum cryptography and violations of Bell's inequalities is discussed.
Network Bell experiments give rise to a form of quantum nonlocality that conceptually goes beyond Bell's theorem. We investigate here the simplest network, known as the bilocality scenario. We depart from the typical use of the Bell state measurement in the network central node and instead introduce a family of symmetric isoentangled measurement bases that generalize the so-called ``elegant joint measurement.'' This leads us to report noise-tolerant quantum correlations that elude bilocal variable models. Inspired by these quantum correlations, we introduce network Bell inequalities for the bilocality scenario and show that they admit noise-tolerant quantum violations. In contrast to many previous studies of network Bell inequalities, neither our inequalities nor their quantum violations are based on standard Bell inequalities and standard quantum nonlocality. Moreover, we pave the way for an experimental realization by presenting a simple two-qubit quantum circuit for the implementation of the elegant joint measurement and our generalization.
Do experiments based on superconducting loops segmented with Josephson junctions (e.g., flux qubits) show macroscopic quantum behavior in the sense of Schr\odinger's cat example? Various arguments based on microscopic and phenomenological models were recently adduced in this debate. We approach this problem by adapting --to flux qubits-- the framework of large-scale quantum coherence, which was already successfully applied to spin ensembles and photonic systems. We show that contemporary experiments might show quantum coherence more than one hundred times larger than experiments in the classical regime. However, we argue that the often used demonstration of an avoided crossing in the energy spectrum is not sufficient to conclude about the presence of large-scale quantum coherence. Alternative, rigorous witnesses are proposed.
Quantum Cryptography is on the verge of commercial application. One of its greatest limitations is over long distance - secret key rates are low and the longest fibre over which any key has been exchanged is currently 100 km. We investigate the quantum relay, which can increase the maximum distance at which quantum cryptography is possible. The relay splits the channel into sections, and sends a different photon across each section, increasing the signal to noise ratio. The photons are linked as in teleportation, with entangled photon pairs and Bell measurements. We show that such a scheme could allow cryptography over hundreds of kilometers with today's detectors. It could not, however, improve the rate of key exchange over distances where the standard single section scheme already works. We also show that reverse key reconciliation, previously used in continuous variable quantum cryptography, gives a secure key over longer distances than forward key reconciliation.
