Abstract
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.
Highlights
It is well understood that the application of quantum mechanics to traditional technology-related problems may give a new twist to a number of fields
We develop the theory for performing a two-way covert quantum communication protocol in the low-frequency regime, in the case where one party has a severe energy constraint
While the results of this paper are quite general, we focus mainly on the 1–10 GHz spectrum, where circuit QED (cQED) platforms have been highly developed in recent decades
Summary
It is well understood that the application of quantum mechanics to traditional technology-related problems may give a new twist to a number of fields. We use a one-time-pad cipher to allow for the covert channel to be measured only by the intended recipients (detectability), and to make the transmitted and reflected signals uncorrelated between each other and indistinguishable from thermal noise (indistinguishability) In this way the protocol is unconditionally secure. We design an entanglement-assisted protocol based on SC states in a cQED setup, which relies solely on Jaynes-Cumming interactions and qubit measurements This is an important requirement for a cQED implementation, since a receiver based on photon detection is currently too demanding to be realistic. VI, we discuss a cQED protocol based on SC states, with a receiver design, which uses Jaynes-Cumming interactions and qubit measurements
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