Two-way covert microwave quantum communication

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 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. Security is reached by covering the carrier signal through the presence of the thermal noise in the environment. We find the ultimate bounds for the receiver performance under different assumptions, proving that quantum correlations can enhance the signal-to-noise ratio by up to $6$ dB using a collective strategy, and $3$ dB with local strategies. We extend the standard square-root law for one-way covert quantum communication to the two-way setup, proving that $O(\sqrt{n})$ number of bits can be reliably transmitted over $n$ channel usages. We show how to engineer all the stages of the entanglement-assisted version of the protocol, by using Jaynes-Cumming interactions and qubit measurements. Our proposal makes a decisive step toward implementing quantum communication concepts in the previously uncharted $1-10$ GHz frequency range.