Encryption is not sufficient when the sole fact that two parties are communicating can be incriminating to them. In such cases, we require methods of covert communication, where an eavesdropper cannot even detect that the communication is taking place. In this work, we extend covert communication into the quantum regime by showing that covert quantum communication is possible over optical channels with noise arising from thermal background radiation, either form the environment or from the sender's lab. In particular, we show that sequences of qubits can be transmitted covertly by using both a single-photon and a coherent-state encoding.
A consequence of these results is that it is possible to carry out covert quantum key distribution using standard optical equipment. We show that covert QKD protocols using sequences of qubits consume more secret key than they produce, and propose instead a hybrid approach that uses pseudo-random number generators (PRNGs) together with covert QKD to regenerate secret keys. The security of the new key is guaranteed by QKD while the security of the covert communication is at least as strong as the security of the PRNG.