The surprising persistence of time-dependent quantum entanglement

TL;DR

This paper demonstrates that time-dependent quantum entanglement in nonlinear interferometry can be preserved and recovered despite experimental imperfections like pump impurity, noise, and dispersion, challenging previous assumptions about entanglement fragility.

Contribution

The study introduces a positive-P approach to show how real-world effects do not necessarily destroy measurable entanglement in quantum nonlinear interferometry.

Findings

Time-domain entanglement remains intact despite pump impurity and noise.

Entanglement can be recovered even with group velocity mismatches.

Experimental complications do not preclude observable quantum entanglement.

Abstract

The mismatch between elegant theoretical models and the detailed experimental reality is particularly pronounced in quantum nonlinear interferometry (QNI). In stark contrast to theory, experiments contain pump beams that start in impure states and that are depleted, quantum noise that affects -- and drives -- any otherwise gradual build up of the signal and idler fields, and nonlinear materials that are far from ideal and have a complicated time-dependent dispersive response. Notably, we would normally expect group velocity mismatches to destroy any possibility of measurable or visible entanglement, even though it remains intact -- the mismatches change the relative timings of induced signal-idler entanglements, thus generating "which path" information. Using a "positive-P" approach ideally suited to such problems, we show how the time-domain entanglement crucial for QNI can be -- and is -- recoverable despite the obscuring effects of real-world complications.