Communicating covertly goes quantum

Coded messages can keep your secrets safe, but you also might prefer to conceal the fact that you’re communicating at all. Now, scientists are working to make even the whispers of top-secret communications completely undetectable by eavesdroppers. And the methods for sending such stealthy dispatches are being extended from ordinary “classical” messages to quantum ones.

The typical method for sending secret messages is encryption, which allows two parties to exchange coded information that a bystander can’t interpret. But if you want to send a message that no one can even tell you’re sending, you need something else: covert communication. “What covertness gives you is a much more secure way of communicating,” says quantum information researcher Boulat Bash of Raytheon BBN Technologies in Cambridge, Mass.

Scientists have already demonstrated covert communication in the classical realm. But a paper posted April 19 on arXiv.org shows that covert communication can be expanded to the quantum world, opening up the possibility of covertly exchanging quantum bits, or qubits.

“It’s a pretty cool step,” says information theorist Matthieu Bloch of Georgia Tech in Atlanta. The result suggests the possibility of combining covert communication with quantum communication procedures, such as quantum cryptography, an ultrasecure method of encryption.

To communicate covertly, messengers send information encoded in a sequence of photons —particles of light. Messengers send these photons through a channel, like a fiber-optic cable.  But they are mixed in with a sea of random photons that don’t encode a message, so eavesdroppers can’t tell if the photons they detect are part of a message or just the usual random fluctuations. The messengers agree ahead of time on the precise moments to send and receive their photons, so they can read the missive but an eavesdropper can’t.

Communication over a classical version of such a covert channel is undetectable, Bash and colleagues showed in a paper in Nature Communications in October 2015. Even the most powerful sleuth, equipped with a futuristic quantum computer, can’t determine whether two parties are communicating. “It’s secure against an adversary who is as powerful as nature can really become,” Bash says.

The new paper extends Bash’s method to the quantum domain, with calculations that show the procedure works even when photons’ properties may be linked through entanglement, or can be in multiple states at once. “There’s nothing about quantum communication that prevents us from doing it covertly,” says Juan Miguel Arrazola of the Centre for Quantum Technologies in Singapore, a coauthor of the new paper.

But, Bloch says, the new study considers only one type of quantum communication channel and relies on some assumptions. For instance, the researchers assume that the eavesdropper doesn’t tamper with the random jumble of photons. “We’re not quite to the point where we can claim that we know how to do covert quantum communication in the general sense,” he says.

Scientists are now hashing out the capabilities of such covert channels. To avoid tipping off a snoop that there’s a message hidden in the mishmash of photons, the messengers can’t send their communiqués as quickly as they otherwise would. This limits the capacity of the channel for covert communications. The next step is to determine whether the same limit holds for all covert quantum communication channels.