  Spoofing an electromagnetic signal involves measuring its properties and
preparing a spoof signal that is a close enough copy to fool a receiver. A
classic application of spoofing is in radar where an airborne target attempts
to avoid being tracked by a ground-based radar by emitting pulses indicating a
false range or velocity. In certain scenarios it has been shown that a sensor
can exploit quantum mechanics to detect spoofing at the single-photon level.
Here we analyze an idealized spoofing scenario where a transmitter-receiver
pair, seeking to detect spoofing, utilizes a signal chosen randomly from a set
of non-orthogonal, coherent states. We show that a spoofer optimally employing
classical information on the state of the transmitted signal (i.e. the best
measure-and-prepare strategy allowed by quantum mechanics) inevitably emits
imperfect spoofs that can be exploited by the receiver to reveal the presence
of the spoofer, or to discriminate between true reflections and spoofs.
Importantly, we show that the quantum limitations on classical spoofing remain
significant even in the large mean-photon-number regime.
