  A new paradigm for secure communication, based on quantum illumination, is
proposed. Alice uses spontaneous parametric down-conversion to send Bob a set
of signal modes over a pure-loss channel while retaining the set of idler modes
with which they are maximally entangled. Bob imposes a single information bit
on the modes he receives from Alice via binary phase-shift keying. He then adds
classical Gaussian noise and sends the noisy modulated modes back to Alice over
the same pure-loss channel. Even though the loss and noise destroy any
entanglement between the modes that Alice receives from Bob and the idler modes
she has retained, she can decode Bob's bit with an error probability that can
be orders of magnitude lower than what is achieved by a passive eavesdropper
who receives all the photons that are lost en route from Alice to Bob and from
Bob to Alice.
