Building High Fidelity Coherent Encoded States for Stealthy Optical Communications Evaluated at the Quantum Limit

Quantum steganography is a powerful method for information security where communications between a sender and receiver are disguised as naturally occurring noise in a channel. We encoded the phase and amplitude of weak coherent laser states such that a third party monitoring the communications channel, measuring the flow of optical states through the channel, would see an amalgamation of states indistinguishable from thermal noise light from amplified spontaneous emission (ASE). Then, we inject these encoded coherent states into a filtered-out band previously containing ASE. Using quantum state tomography, we experimentally reconstructed the density matrices for artificially engineered thermal states and spontaneous emission from an optical amplifier and verified a state fidelity F>0.98 when compared with theoretical thermal states. We additionally saw a state fidelity F>0.97 when comparing the two experimentally reconstructed states at different mean photon numbers. Finally, we will present a communication demonstration transmitting a 100x100 pixel image using the quantum steganography protocol.