Quantum nonseparability: How the problem evolved from Einstein's time to ours

Tullio Weber

Anna Napoli, M. Guccione, Antonio Messina +1 more

We show that two interacting physical systems may admit entangled pure or nonseparable mixed states evolving in time as if the mutual interaction Hamiltonian were absent. In this paper we define these interaction-free evolving (IFE) states and characterize their existence for a generic binary system described by a time-independent Hamiltonian. A comparison between IFE subspace and the decoherence-free subspace is reported. The set of all pure IFE states is explicitly constructed for a nonhomogeneous spin-star-system model

Asher Peres

Bernard d’Espagnat

As a matter of fact, no serious description of the Einstein-Podolsky-Rosen problem uses the incriminated postulate that the state vector is objective. The most straightforward case is that of realism. Realism, however, is not universally accepted. In Neil Bohr's philosophy, nonseparability is also present, but it has a very different meaning. The nonseparability of the quantum world is thus transferred, in this philosophy, from the system of the microparticles to the systems that each of these micro-objects constitutes with its instrument. In regard to Lorentz invariance, the transition from the pure case to the mixture—the reduction of the wave packet—raises less acute problems in Bohr's philosophy than it does in other approaches. Moreover, Bohr's philosophy obviously implies some restrictions on our freedom to imagine an independent external reality. The reason for this is that this conception endows the instruments with virtues that clearly transcend their status as mere physical systems —indeed, it ultimately defines instruments with reference to the desires.

Vassilios Karakostas

William K. Wootters, Wojciech Hubert Zurek

A detailed analysis of Einstein's version of the double-slit experiment, in which one tries to observe both wave and particle properties of light, is performed. Quantum nonseparability appears in the derivation of the interference pattern, which proves to be surprisingly sharp even when the trajectories of the photons have been determined with fairly high accuracy. An information-theoretic approach to this problem leads to a quantitative formulation of Bohr's complementarity principle for the case of the double-slit experiment. A practically realizable version of this experiment, to which the above analysis applies, is proposed.