  Superposition and entanglement, the quintessential characteristics of quantum
physics, have been shown to provide communication, computation, and sensing
capabilities that go beyond what classical physics will permit. It is natural,
therefore, to explore their application to radar, despite the fact that
decoherence---caused by the loss and noise encountered in radar
sensing---destroys these fragile quantum properties. This paper tells the story
of "quantum illumination", an entanglement-based approach to quantum radar,
from its inception to its current understanding. Remarkably, despite loss and
noise that destroy its initial entanglement, quantum illumination does offer a
target-detection performance improvement over a classical radar of the same
transmitted energy. A realistic assessment of that improvement's utility,
however, shows that its value is severely limited. Nevertheless, the fact that
entanglement can be of value on an entanglement-breaking channel---the
meta-lesson of the quantum illumination story---should spur continued research
on quantum radar.
