Entanglement conditions involving intensity correlations of optical fields: the case of multi-port interferometry

TL;DR

This paper extends the use of normalized quantum Stokes operators to multi-mode bright squeezed vacuum states, providing more noise-resistant entanglement conditions applicable to multi-port interferometry and EPR-like experiments.

Contribution

It introduces new entanglement conditions based on normalized intensity correlations that are more robust against photon-loss noise in multi-mode optical systems.

Findings

Enhanced entanglement detection in noisy environments.

Applicable to prime number modes in multi-port interferometry.

More noise-resistant than standard intensity-based methods.

Abstract

Normalized quantum Stokes operators introduced in [Phys. Rev. A {\bf 95}, 042113 (2017)] enable one to better observe non-classical correlations of entangled states of optical fields with undefined photon numbers. For a given run of an experiment the new quantum Stokes operators are defined by the differences of the measured intensities (or photon numbers) at the exits of a polarizer divided by their sum. It is this ratio that is to be averaged, and not the numerator and the denominator separately, as it is in the conventional approach. The new approach allows to construct more robust entanglement indicators against photon-loss noise, which can detect entangled optical states in situations in which witnesses using standard Stokes operators fail. Here we show an extension of this approach beyond phenomena linked with polarization. We discuss EPR-like experiments involving correlations produced by optical beams in a multi-mode bright squeezed vacuum state. EPR-inspired entanglement conditions for all prime numbers of modes are presented. The conditions are much more resistant to noise due to photon loss than similar ones which employ standard Glauber-like intensity, correlations.