Sensing has been in the centre of interest of the quantum information community in the last years. The main reason is that quantum mechanics allow for enhanced precision and the foremost focus has been to find optimal quantum probe states and measurements to attain the quantum enhanced precision.

In their recent work (DOI: 10.1103/PhysRevA.99.062321), Christos (Arizona, formerly at Warwick), Animesh, and colleagues from the University of Arizona, unlock another kind of feature: active covert sensing. The key element of covert sensing is that the sensing light can be hidden in the thermal environment. Specifically, it is shown that it is possible to sense a phase while an adversary remains unaware of the sensing process and they give the fundamental limit: The mean square error of any covert sensing task is lower bounded by the inverse square root of the probe's number of modes (or the number sensing attempts). Any attempt of the sensor to improve the precision necessarily leads to detection by the adversary.

In this work, it is proven that floodlight illumination, i.e., correlated thermal light from an amplified spontaneous emission source, can outperform a coherent light source due to the source's larger optical bandwidth. Heterodyne detection is proven to suffice to attain the fundamental covert sensing scaling. Free space covert sensing is studied and it is found that sources in the mid-long infrared range are optimal.