Covert Quantum Communication Over Optical Channels

TL;DR

This paper demonstrates that it is possible to covertly transmit a number of qubits proportional to the square root of the number of channel uses over optical channels, using quantum noise concealment techniques.

Contribution

It establishes a quantum analogue of the classical square root law for covert communication over optical channels, with new achievability and converse proofs.

Findings

Achieves covert quantum communication with  qubits over n channel uses.

Uses photonic dual-rail qubit encoding for covert transmission.

Bounds the number of covert qubits, suggesting room for improvement.

Abstract

We explore covert communication of qubits over the lossy thermal-noise bosonic channel, which is a quantum-mechanical model of many practical channels, including optical. Covert communication ensures that an adversary is unable to detect the presence of transmissions, which are concealed in channel noise. We show a \emph{square root law} (SRL) for quantum covert communication similar to that for classical: $\propto\sqrt{n}$ qubits can be transmitted covertly and reliably over $n$ uses of an optical channel. Our achievability proof uses photonic dual-rail qubit encoding, which has been proposed for long-range repeater-based quantum communication and entanglement distribution. Our converse employs prior covert signal power limit results and adapts well-known methods to upper bound quantum capacity of optical channels. Finally, we believe that the gap between our lower and upper bounds for the number of reliable covert qubits can be mitigated by improving the quantum error correction codes and quantum channel capacity bounds.