Discrimination of discord in separable Gaussian states

TL;DR

This paper compares the effectiveness of joint coherent measurements versus local Gaussian measurements in discriminating between thermal and separable Gaussian states with quantum discord, showing the superiority of the former.

Contribution

It demonstrates that local Gaussian measurements are strictly suboptimal for detecting quantum discord compared to joint coherent measurements.

Findings

Coherent measurements outperform local Gaussian measurements in state discrimination.

Local Gaussian measurements are insufficient for optimal discord detection.

Non-Gaussian or non-local measurements could improve detection performance.

Abstract

Consider two bosonic modes which are prepared in one of two possible Gaussian states with the same local energy: either a tensor-product thermal state (with zero correlations) or a separable Gaussian state with maximal correlations (with both classical and quantum correlations, the latter being quantified by quantum discord). For the discrimination of these states, we compare the optimal joint coherent measurement with the best local measurement based on single-mode Gaussian detections. We show how the coherent measurement always strictly outperforms the local detection strategy for both single- and multi-copy discrimination. This means that using local Gaussian measurements (assisted by classical communication) is strictly suboptimal in detecting discord. A better performance may only be achieved by either using non Gaussian measurements (non linear optics) or coherent non-local measurements.