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arXiv:quant-ph/0602005

Quantum correlations in successive spin measurements

Ali Ahanj, Pramod S. Joag

Published Feb 1, 2006• 0 citations
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Abstract

In this paper we present a new approach for testing QM against the realism aspect of hidden variable theory (HVT). We consider successive measurements of non-commuting operators on a input spin $s$ state. The key point is that, although these operators are non-commuting, they act on different states so that the joint probabilities for the outputs of successive measurements are well defined. We show that, in this scenario HVT leads to Bell type inequalities for the correlation between the outputs of successive measurements. We account for the maximum violation of these inequalities by quantum correlations by varying spin value and the number of successive measurements. Our approach can be used to obtain a measure of the deviation of QM from realism say in terms of the amount of information needed to be transferred between successive measurements in order to classically simulate the quantum correlations.

DOI
10.48550/arxiv.quant-ph/0602005

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In this paper we present a new approach for testing QM against the realism aspect of hidden variable theory (HVT). We consider successive measurements of non-commuting operators on a input spin sss state. The key point is that, although these operators are non-commuting, they act on different states so that the joint probabilities for the outputs of successive measurements are well defined. We show that, in this scenario HVT leads to Bell type inequalities for the correlation between the outputs of successive measurements. We account for the maximum violation of these inequalities by quantum correlations by varying spin value and the number of successive measurements. Our approach can be used to obtain a measure of the deviation of QM from realism say in terms of the amount of information needed to be transferred between successive measurements in order to classically simulate the quantum correlations.
In this paper, we consider a hidden variable theoretical description of successive measurements of non-commuting spin observables on an input spin-s state. Although these spin observables are non-commuting, they act on different states, and so the joint probabilities for the outputs of successive measurements are well-defined. We show that, in this scenario, hidden variable theory (HVT) leads to Bell-type inequalities for the correlation between the outputs of successive measurements. We account for the maximum violation of these inequalities by quantum correlations (i.e. the correlations of successive measurements on a quantum state) by varying the spin value and the number of successive measurements. Our approach can be used to obtain a measure of the deviation of Quantum Mechanics from the theory obeying realism and time-locality, in terms of the amount of classical information needed to be transferred between successive measurements in order to simulate the above-mentioned correlations in successive measurements.
Bell’s inequality is an expression which can be employed in order to test for the existence of hidden variables in a correlated system. The idea of hidden variables was introduced as a potential explanation for the quantum mechanical phenomenon of non-deterministic results of measurements. The existence of hidden variables can be tested experimentally. First, a particular measurement of one component of a correlated system must be associated with a positive or negative integer value. There must be an equal probability density of obtaining either measurement. Following this, an ensemble average of these measurements must be performed and Bell’s inequality must subsequently be employed. This theorem was essential in settling the EPR argument against quantum mechanical indeterminism, experimentally showing through violation of the inequality that there are indeed no hidden variables in a quantum system. For a system exhibiting classical behavior, Bell’s inequality should not be violated, provided the correlated system meets criteria appropriate to the assumptions underlying Bell’s theorem. A Mathematica model of Bell’s inequality was developed utilizing a Monte Carlo approach of randomizing parameters in order to test models of correlated systems. The model appears to agree with classical determinism within one standard deviation. It is possible to test both classical and quantum mechanical systems if properly correlated, and it is of future interest to test whether this extends to chaotic systems.
Cooperative multiparticle quantum states in an Einstein-Podolsky-Rosen type of experiment are shown to violate a Bell inequality. Thus correlations can occur between wave packets each having NNN bosons which cannot be explained within local hidden-variable theories. This provides a way to test quantum measurement theory for multiparticle states. In these states, a local, positive probability distribution exists for all NNN. However, the representation of correlations with this probability distribution is different from that in standard (Bell-type) hidden-variable theories.
Quantum non-locality tests have been of interest since the original EPR paper. The present paper discusses whether the CGLMP (Bell) inequalities obtained by Collins et al are possible tests for showing that quantum theory is not underpinned by local hidden variable theory (LHVT). It is found by applying Fine's theorem that the CGLMP approach involves a LHVT for the probabilities associated with measurements on two observables (each from one of the two sub-subsystems), even though the underlying probabilities for measurements of all four observables involve a hidden variable theory which is not required to be local. Although the CGLMP inequalities involve probabilities for measurements of one observable per sub-system and are compatible with the Heisenberg uncertainty principle, there is no unambiguous quantum measurement process linked to the probabilities in the CGLMP inequalities. Quantum measurements corresponding to the different classical measurements that give the same CGLMP probability are found to yield different CGLMP probabilities. However, violation of a CGLMP inequality based on any one of the possible quantum measurement sequences is sufficient to show that the Collins et al LHVT does not predict the same results as quantum theory. This is found to occur for a state considered in their paper - though for observables whose physical interpretation is unclear. In spite of the problems of comparing the HVT inequalities with quantum expressions, it is concluded that the CGLMP inequalities are indeed suitable for ruling out local hidden variable theories. The state involved could apply to a macroscopic system, so the CGLMP Bell inequalities are important for finding cases of macroscopic violations of Bell locality. Possible experiments in double-well Bose condensates involving atoms with two hyperfine components are discussed.
In the derivation of Bell's inequalities, probability distribution is supposed to be a function of only hidden variable. We point out that the true implication of the probability distribution of Bell's correlation function is the distribution of the joint measurement outcomes on the two sides. So it is a function of both hidden variable and settings. In this case, Bell's inequalities fail. Our further analysis shows that Bell's locality holds neither for dependent events nor for independent events. We think that the measurements of EPR pairs are dependent events, thus violation of Bell's inequalities cannot rule out the existence of local hidden variable. In order to explain the results of EPR-type experiments, we suppose that polarization entangled photon pair can be composed of two circularly or linearly polarized photons with correlated hidden variables, and a couple of experiments of quantum measurement are proposed. The first uses delayed measurement on one photon of the EPR pair to demonstrate directly whether measurement on the other could have any non-local influence on it. Then several experiments are suggested to reveal the components of polarization entangled photon pair. The last one uses successive polarization measurements on a pair of EPR photons to show that two photons with a same quantum state will behave in the same way under the same measuring condition.