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arXiv:2405.03881 · IEEE Transactions on Quantum Engineering

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

Haley A. Weinstein, Bruno Avritzer, Christine M. Kinzfogl, Todd A. Brun, Jonathan L. Habif

Published Jan 1, 2025• 0 citations

Abstract

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$F=0.98$when compared with theoretical thermal states.

DOI: 10.1109/tqe.2025.3596491

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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

F=0.98

when compared with theoretical thermal states.

Steganography is an alternative to cryptography, where information is protected by secrecy (being disguised as innocent communication or noise) rather than being scrambled. In this work we develop schemes for steganographic communication using Fock and coherent states in optical channels based on disguising the communications as thermal noise, with a fidelity approaching 1. We derive bounds on their efficiency in terms of the communication rate and the required keyrate in the case of an all-powerful eavesdropper and provide explicit methods of encoding and error correction for the noiseless channel case.

In the Gaussian-modulated coherent state quantum key distribution (QKD) protocol, the sender first generates Gaussian-distributed random numbers and then encodes them on weak laser pulses actively by performing amplitude and phase modulations. Recently, an equivalent passive QKD scheme has been proposed by exploring the intrinsic field fluctuations of a thermal source [B. Qi, P. G. Evans, and W. P. Grice, Phys. Rev. A 97, 012317 (2018)]. This passive QKD scheme is especially appealing for chip-scale implementation since no active modulation is required. In this paper, we conduct an experimental study of the passively encoded QKD scheme using an off-the-shelf amplified spontaneous emission source operated in continuous-wave mode. Our results show that the excess noise introduced by the passive state preparation scheme can be effectively suppressed by applying optical attenuation and a secure key can be generated over metro-area distances.

We consider discrete-alphabet encoding schemes for coherent-state quantum key distribution. The sender encodes the letters of a finite-size alphabet into coherent states whose amplitudes are symmetrically distributed on a circle centered in the origin of the phase space. We study the asymptotic performance of this phase-encoded coherent-state protocol in direct and reverse reconciliation assuming both loss and thermal noise in the communication channel. In particular, we show that using just four phase-shifted coherent states is sufficient for generating secret-key rates of the order of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ bits per channel use at about 15 dB loss in the presence of realistic excess noise.