Local Realistic Models Research Articles

Computational Power of Correlations

We study the intrinsic computational power of correlations exploited in measurement-based quantum computation. By defining a general framework, the meaning of the computational power of correlations is made precise. This leads to a notion of resource states for measurement-based classical computation. Surprisingly, the Greenberger-Horne-Zeilinger and Clauser-Horne-Shimony-Holt problems emerge as optimal examples. Our work exposes an intriguing relationship between the violation of local realistic models and the computational power of entangled resource states.

Violation of Rotational Invariance of Local Realistic Models with Two Settings

We have considered a two-particle Bell experiment to visualize the conflict between rotational invariance of physical laws and a specific local realistic theory. The experiment is reproducible by using a local realistic theory obtained in a two-setting Bell experiment. The generalized Bell inequality {[J. Phys. A: Math. Theor. {\bf 40}, 13101 (2007)]}, which is derived under the assumption that there exists a rotationally invariant local realistic theory, turns out to disprove such a local realistic model existing with a two-setting experiment. This implies that such a model is not rotationally invariant and should, therefore, be ruled out in some situations.

Measurement of Einstein-Podolsky-Rosen-Type Flavor Entanglement inΥ(4S)→B0B¯0Decays

The neutral B-meson pair produced at the Upsilon(4S) should exhibit a non-local correlation of the type discussed by Einstein, Podolski, and Rosen (EPR). We measure this correlation using the time-dependent flavour asymmetry of semileptonic B0 decays, which we compare with predictions from quantum mechanics and two local realistic models. The data are consistent with quantum mechanics, and inconsistent with the other models. Assuming that some B pairs disentangle to produce B0 and B0bar with definite flavour, we find a decoherent fraction of 0.029+-057, consistent with no decoherence.

Comment on “A local realist model for correlations of the singlet state” by K. De Raedt, K. Keimpema, H. De Raedt, K. Michielsen and S. Miyashita

De Raedt et al. (Eur. Phys. J. B 53: 139-142, 2006) have claimed to provide a local realist model for correlations of the singlet state in the familiar Einstein-Podolsky-Rosen-Bohm (EPRB) experiment when time-coincidence is used to decide which detection events should count in the analysis, and furthermore that this suggests that it is possible to construct local realistic models that can reproduce the quantum mechanical expectation values. In this letter we show that these conclusions cannot be upheld since their model exploits the so-called coincidence-time loophole. When this is properly taken into account no startling conclusions can be drawn about local realist modelling of quantum mechanics.

Experimental local realism tests without fair sampling assumption

Following the theoretical suggestion of [E. Santos, Phys. Lett. A 327, 33 (2004); E. Santos, Eur. Phys. J. D 42, 501 (2007)], we present experimental results addressed to test restricted families of local realistic models, but without relying on the fair sampling assumption.

Separability of Quantum States vs. Original Bell (1964) Inequalities

All separable states satisfy all Bell-type inequalities, which involve as their assumption only existence of local realistic (local hidden variable) models of the correlations of spatially separated systems, observed by two or more observers making independent decisions on what to measure (free will). The recent observation by Loubenets, that some separable states do not satisfy the original Bell inequality (1964) has no consequences whatsoever for the studies of the relation of separability with local realism. The original Bell inequality was derived using an additional assumption that the local results for a certain pair of local settings reveal perfect Einstein–Podolsky–Rosen (EPR) correlations. Therefore violation of this inequality by some quantum predictions implies that either (i) the predictions do not allow a local realistic model, or (ii) the predictions do not have the required EPR correlations, or finally both (i) and (ii).

RotationalInvariance as an Additional Constraint on Local Realism

Rotational invariance of physical laws is a generally accepted principle. We show that it leads to an additional external constraint on local realistic models of physical phenomena involving measurements of multiparticle spin 1/2 correlations. This new constraint rules out such models even in some situations in which standard Bell inequalities allow for explicit construction of such models. The whole analysis is performed without any additional assumptions on the form of local realistic models.

On Bell's theorem for N‐qubits

AbstractWe present a single general Bell inequality that summarizes all possible Bell type constrains on the correlation function for the N‐particle system (i.e., it is a necessary and sufficient condition for the existence of local realistic models for the correlation function). It is applicable in the case when measurements on each particle can be chosen between two arbitrary dichotomic observables. We also obtain a necessary and sufficient condition for an arbitrary N‐qubit mixed state to violate this general Bell inequality.

Local Realistic Models Tested by the EPRB Experiment with Random Variable Analyzers

We carry out an analysis of the output data files of the Einstein-Podolsky-Rosen-Bohm experiment realized with random variable analyzers (Innsbruck, 1998). That experiment was devised to disprove Local Realistic alternatives (to Quantum Mechanics) based on the predictability of the analyzers’ positions. The disproval is not immediate, for the quantum mechanical results can be reproduced even if the probability of guessing the analyzers’ position is small. Our analysis is aimed to determine, in quantitative terms, whether this experiment was able to disprove Local Realism. We conclude that the experiment did disprove Local Realism. In addition, the experiment provides time-resolved data files which allow us to look for privileged frequencies in the spectrum of the time fluctuations. Such privileged frequencies, if existed, could be interpreted as a sign of a non-ergodic behavior. To our knowledge, this is the first time such a study is proposed and performed.

Generalized Bell inequality in two neutral kaon systems

We present a contradiction between quantum mechanics and local realistic models, Bell inequalities, for correlated two neutral kaon systems. They predict different values for real observables. The magnitude of violation of the inequality in this gedanken experiment is of order |ε|, the CP violating parameter.

Quantum mechanics versus local realism: the importance of the wave function normalization

Quantum mechanical predictions for physical correlated systems can be obtained from hidden-variable local realistic models if the quantum operation of normalization of the probability amplitudes is imposed. Thus it is stressed the critical role of the mathematical operation of normalization in order to jump from a realistic and local description of the microscopical correlated physical world to a quantum mechanical one, giving rise to the EPR paradox.

Local realistic models and non-physical probabilities

We give a general argument showing that one can always reproduce the quantum mechanical results for non-factorisable state vectors of correlated systems by hidden variable models using non-physical probabilities, as recently done by Six for the particular case of correlated K 0− K 0 pairs.

Polarization correlation analysis of the radiation from a two-photon deuterium source using three polarizers: A test of quantum mechanics versus local realism.

Measurements have been made of the polarization properties of the two photons emitted in the decay of metastable atomic deuterium in an experiment in which three linear polarizers, instead of the normal two, are used. The quantum-mechanical predictions for these imperfect polarizers have been calculated and shown to be in good agreement with the experimental results. The results have also been used to test local realistic models which are capable of explaining all existing two-polarizer-type experiments, and have provided conclusive evidence against these theories.

Local realistic photon models and EPR-type experiments

From a wide class of local photon models which satisfy two natural physical conditions a new inequality is deduced which is violated in the usual quantum theory. No additional assumptions of the CHSH- and/or CH-type are made. The present results are compared with those recently obtained by Marshall and Santos.

Enhanced photon detection in EPR type experiments

Local realistic models of enhanced photon detection reproducing quantum mechanical predictions for single photons and approximating closely the quantum mechanical predictions for pairs of correlated photons are discussed. It is shown that Einstein-Podolsky-Rosen-type experiments with three polarizers can easily discriminate such models from ordinary quantum theory.