Spin entanglement, decoherence and Bohm's EPR paradox

TL;DR

This paper establishes criteria for spin entanglement and the EPR paradox, analyzing how decoherence affects their observability in photonic and macroscopic systems, with implications for quantum communication.

Contribution

It provides new thresholds for entanglement and EPR paradox detection considering decoherence effects, applicable to both microscopic and macroscopic quantum systems.

Findings

Entanglement can occur at all transmission efficiencies if state purity is above a threshold.

Bohm's EPR paradox requires a minimum efficiency of 58%.

Macroscopic entanglement and EPR paradox are robust above efficiencies of 1/3 and 2/3, respectively.

Abstract

We obtain criteria for entanglement and the EPR paradox for spin-entangled particles and analyse the effects of decoherence caused by absorption and state purity errors. For a two qubit photonic state, entanglement can occur for all transmission efficiencies. In this case, the state preparation purity must be above a threshold value. However, Bohm's spin EPR paradox can be achieved only above a critical level of loss. We calculate a required efficiency of 58%, which appears achievable with current quantum optical technologies. For a macroscopic number of particles prepared in a correlated state, spin entanglement and the EPR paradox can be demonstrated using our criteria for efficiencies {\eta} > 1/3 and {\eta} > 2/3 respectively. This indicates a surprising insensitivity to loss decoherence, in a macroscopic system of ultra-cold atoms or photons.