Quantum limits of covert target detection

In covert target detection, Alice attempts to send optical or microwave probes to detect whether or not a weakly-reflecting target embedded in thermal background radiation is present in a target region while remaining undetected herself by an adversary Willie who is co-located with the target and collects all the light that does not return to Alice. We formulate this problem in a realistic setting and derive quantum-mechanical limits on Alice's error probability performance in entanglement-assisted target detection for any fixed level of her detectability by Willie. In particular, we show that Alice must expend a minimum energy in her probe light to maintain a given covertness level, but is also able to achieve a nonzero error probability exponent while remaining perfectly covert. We compare the performance of two-mode squeezed vacuum probes and Gaussian-distributed coherent states to our performance limits. We also obtain quantum limits for discriminating any two thermal loss channels and for non-adversarial quantum illumination without the no-passive-signature assumption.Comment: 18 pages, 5 figure