High-Fidelity Artificial Quantum Thermal State Generation Using Encoded Coherent States

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. A candidate resource state required for implementing quantum steganography is a weak coherent state engineered with modulated phase and amplitude values drawn from probability distributions that result in a mixed state indistinguishable from a thermal state. We experimentally demonstrate construction of this resource state by encoding 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 such as that from spontaneous emission. Using quantum state tomography, we experimentally reconstructed the density matrices for the artificially engineered thermal states and spontaneous emission from an optical amplifier and verified a mean state fidelity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$F=0.98$</tex-math></inline-formula> when compared with theoretical thermal states.