  In covert target detection, Alice attempts to send optical or microwave
probes to determine the presence or absence of a weakly-reflecting target
embedded in thermal background radiation within a target region, while striving
to remain undetected by an adversary, Willie, who is co-located with the target
and collects all 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. We demonstrate how Alice can approach
this performance limit using two-mode squeezed vacuum probes in the regime of
small to moderate background brightness, and how such protocols can outperform
any conventional approach using Gaussian-distributed coherent states. In
addition, we derive a universal performance bound for non-adversarial quantum
illumination without requiring the passive-signature assumption.
