  Quantum communication addresses the problem of exchanging information across
macroscopic distances by employing encryption techniques based on quantum
mechanical laws. Here, we advance a new paradigm for secure quantum
communication by combining backscattering concepts with covert communication in
the microwave regime. Our protocol allows communication between Alice, who uses
only discrete phase modulations, and Bob, who has access to cryogenic microwave
technology. Using notions of quantum channel discrimination and quantum
metrology, we find the ultimate bounds for the receiver performance, proving
that quantum correlations can enhance the SNR by up to $6$ dB. These bounds
rule out any quantum illumination advantage when the source is strongly
amplified, and show that a relevant gain is possible only in the low
photon-number regime. We show how the protocol can be used for covert
communication, where the carrier signal is indistinguishable from the thermal
noise in the environment. We complement our information-theoretic results with
a feasible experimental proposal in a circuit-QED platform. This work makes a
decisive step toward implementing secure quantum communication concepts in the
previously uncharted $1$-$10$ GHz frequency range, in the scenario when the
disposable power of one party is severely constrained.
