Not even 6 dB: Gaussian quantum illumination in thermal background

TL;DR

This paper reevaluates the performance limits of Gaussian quantum illumination in thermal backgrounds, revealing that the commonly claimed 6 dB advantage is unachievable in realistic scenarios and that weak squeezing can be detrimental.

Contribution

It introduces a target-agnostic thermal environment model, challenging previous assumptions and clarifying the true limitations of quantum illumination in practical settings.

Findings

The 6 dB quantum advantage is an unachievable limit in realistic thermal backgrounds.

Weak single-mode squeezing can perform worse than no illumination.

Quantum illumination performance depends critically on background assumptions.

Abstract

In analyses of target detection with Gaussian state transmitters in a thermal background, the thermal occupation is taken to depend on the target reflectivity in a way which simplifies the analysis of the symmetric quantum hypothesis testing problem. However, this assumption precludes comparison of target detection performance between an arbitrary transmitter and a vacuum state transmitter, i.e., ``detection without illumination'', which is relevant in a bright thermal background because a target can be detected by its optical shadow or some other perturbation of the background. Using a target-agnostic thermal environment leads to the result that the oft-claimed 6 dB possible reduction in the quantum Chernoff exponent for a two-mode squeezed vacuum transmitter over a coherent state transmitter in high-occupation thermal background is an unachievable limiting value, only occurring in a limit in which the target detection problem is ill-posed. Further analyzing quantum illumination in a target-agnostic thermal environment shows that a weak single-mode squeezed transmitter performs worse than ``no illumination'', which is explained by the noise-increasing property of reflected low-intensity squeezed light.