Abstract
Encryption prevents unauthorized decoding, but does not ensure stealth – a security demand that a mere presence of a message be undetectable. We characterize the ultimate limit of covert communication that is secure against the most powerful physically-permissible adversary. We show that, although it is impossible over a pure-loss channel, covert communication is attainable in the presence of any excess noise, such as a $300$K thermal blackbody. In this case, $\mathcal{O}(\sqrt{n})$ bits can be transmitted reliably and covertly in $n$ optical modes using standard optical communication equipment, such as a laser-light transmitter and a homodyne receiver. The all-powerful adversary may intercept all transmitted photons not received by the intended receiver, and employ arbitrary quantum memory and measurements. Conversely, we show that this square root scaling cannot be outperformed. We corroborate our theory in a proof-of-concept experiment. We believe that our findings will enable practical realizations of covert communication and sensing, both for point-to-point and networked scenarios.

Biosketch

Boulat Bash received his B.A. in Economics from Dartmouth College in Hanover, New Hampshire in 2001, and his M.S. in Computer Science from the University of Massachusetts at Amherst in 2008. He is currently pursuing his Ph.D. in Computer Science from the University of Massachusetts, Amherst. He spent the summer of 2013 as a visiting scientist with the Quantum Information Processing Group at Raytheon BBN Technologies in Cambridge, Massachusetts. His research interests include privacy, communications, information theory, and signal processing.