Toward Undetectable Quantum Key Distribution over Bosonic Channels
This addresses the challenge of undetectable secure communication for quantum cryptography users, though it is incremental as it builds on existing QKD protocols.
The paper tackled the problem of enabling covert quantum key distribution (QKD) over bosonic channels, showing that covert secret key expansion is possible using a public classical channel and a quantum channel under adversary control, with a protocol achieving positive throughput for certain channel parameters. It also proved a converse result that no covert key generation is possible under full adversary control of the quantum channel.
We show that covert secret key expansion is possible using a public authenticated classical channel and a quantum channel largely under control of an adversary, which we precisely define. We also prove a converse result showing that, under the golden standard of quantum key distribution by which the adversary completely controls the quantum channel, no covert key generation is possible. We propose a protocol based on pulse-position modulation and multi-level coding that allows one to use traditional quantum key distribution (QKD) protocols while ensuring covertness, in the sense that no statistical test by the adversary can detect the presence of communication better than a random guess. When run over a bosonic channel, our protocol can leverage existing discrete-modulated continuous variable protocols. Since existing techniques to bound Eve's information do not directly apply, we develop a new bound that results in positive throughput for a range of channel parameters.