Experimental Covert Communication Over Metropolitan Fibre Optical Links

Covert communication offers a method to transmit messages in such a way that the communication is undetected. Yet, unconditional secure covert communication has not yet been achieved. In this work, we report an experimental demonstration of covert communication that is provably secure against quantum adversaries. We deploy the protocol in a dense wavelength-division multiplexing infrastructure, where our system coexists with a co-propagating C-band classical channel over 10 km of optical fiber. The noise from the classical channel, generally a negative factor in quantum communication, allows us to perform covert quantum communication in a neighboring channel. We report the transmission of three different messages with varying levels of security. Our results demonstrate the feasibility of secure covert communication in a practical setting, which shall find immediate application in optical communication.