# Covert sensing using floodlight illumination

## TL;DR

This paper introduces a covert active sensing method using broadband ASE sources and heterodyne detection, achieving superior performance by spreading probe light over many modes while remaining undetectable.

## Contribution

It demonstrates that broadband ASE sources enable effective covert sensing with higher performance than narrowband lasers, despite thermal noise limitations.

## Key findings

- ASE source's wide bandwidth improves sensing performance
- Covert sensing remains undetectable under quantum noise constraints
- System is potentially extendable to various optical sensing applications

## Abstract

We propose a scheme for covert active sensing using floodlight illumination from a THz-bandwidth amplified spontaneous emission (ASE) source and heterodyne detection. We evaluate the quantum-estimation-theoretic performance limit of covert sensing, wherein a transmitter's attempt to sense a target phase is kept undetectable to a quantum-equipped passive adversary, by hiding the signal photons under the thermal noise floor. Despite the quantum state of each mode of the ASE source being mixed (thermal), and hence inferior compared to the pure coherent state of a laser mode, the thousand-times higher optical bandwidth of the ASE source results in achieving a substantially superior performance compared to a narrowband laser source by allowing the probe light to be spread over many more orthogonal temporal modes within a given integration time. Even though our analysis is restricted to single-mode phase sensing, this system could be applicable extendible for various practical optical sensing applications.