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Chinese Physics Letters

Construction of the Einstein-Podolsky-Rosen Entangled States

M. Q. Ruan, Zeng Jin-Yan

Published Aug 29, 2003• 2 citations
Publisher

Abstract

As simultaneous eigenstates of a complete set of commuting observables, construction of the Einstein-Podolsky-Rosen entangled states for a two-particle system is investigated and a sufficient condition to ensure the entanglement between a pair of commuting observables is given. It is found that for a two-particle system, four types of entangled states concerning the usually interested observables may be constructed.

DOI
10.1088/0256-307x/20/9/303

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As simultaneous eigenstates of a complete set of commuting observables, construction of the Einstein-Podolsky-Rosen entangled states for a two-particle system is investigated and a sufficient condition to ensure the entanglement between a pair of commuting observables is given. It is found that for a two-particle system, four types of entangled states concerning the usually interested observables may be constructed.
Pairs of atoms have been prepared in an entangled state of the Einstein-Podolsky-Rosen (EPR) type. They were produced by the exchange of a single photon between the atoms in a high QQQ cavity. The atoms, entangled in a superposition involving two different circular Rydberg states, were separated by a distance of the order of 1 cm. At variance with most previous EPR experiments, this one involves massive particles. It can be generalized to three or more atoms and opens the way to new tests of nonlocality in mesoscopic quantum systems.
A generalization of the 1935 Einstein-Podolsky-Rosen (EPR) argument for measurements with continuous variable outcomes is presented to establish criteria for the demonstration of the EPR paradox, for situations where the correlation between spatially separated subsystems is not perfect. Two types of criteria for EPR correlations are determined. The first type are based on measurements of the variances of conditional probability distributions and are necessary to reflect directly the situation of the original EPR paradox. The second weaker set of EPR criteria are based on the proven failure of (Bell-type) local realistic theories which could be consistent with a local quantum description for each subsystem. The relationship with criteria sufficient to prove entanglement is established, to show that any demonstration of EPR correlations will also signify entanglement. It is also shown how a demonstration of entanglement between two spatially separated subsystems, if not able to interpreted as a violation of a Bell-type inequality, may be interpreted as a demonstration of the EPR correlations. In particular it is explained how the experimental observation of two-mode squeezing using spatially separated detectors will signify not only entanglement but EPR correlations defined in a general sense.
In quantum mechanics, the general state describing two or more particles is a linear superposition of product states. Such a superposition is called entangled if it cannot be factored into just one product. When only two particles are entangled, the stage is set for Einstein-Podolsky-Rosen (EPR) discussions and Bell's proof that the EPR viewpoint contradicts quantum mechanics. If more than two particles are involved, new possibilities and phenomena arise. For example, the Greenberger, Horne, and Zeilinger (GHZ) disproof of EPR applies. Furthermore, as we point out, with three or more particles even entanglement itself can be an entangled property.
For finite-dimensional systems, states satisfying perfect correlation are just maximally entangled states that play an essential role in quantum information theory. Thus perfect correlation may be a key to entanglement phenomena. For general systems, Einstein–Podolsky–Rosen type states are characterized by perfect correlation and their properties are discussed. Conditions equivalent to perfect correlation are found. As an application of perfect correlation, all Einstein–Podolsky–Rosen type states on a two-particle system are given. Unlike in finite-dimensional systems, they are generally not quasiequivalent.
The strange property of the Einstein-Podolsky-Rosen (EPR) correlation between two remote physical systems is a primitive object on the study of quantum entanglement. In order to understand the entanglement in canonical continuous-variable systems, a pair of the EPR-like uncertainties is an essential tool. Here, we consider a normalized pair of the EPR-like uncertainties and introduce a state-overlap to a classically correlated mixture of coherent states. The separable condition associated with this state-overlap determines the strength of the EPR-like correlation on a coherent-state basis in order that the state is entangled. We show that the coherent-state-based condition is capable of detecting the class of two-mode Gaussian entangled states. We also present an experimental measurement scheme for estimation of the state-overlap by a heterodyne measurement and a photon detection with a feedforward operation.