Free-Space Quantum Communications in Harsh Environments

Exploring the possibility of all-weather secure quantum communication using macroscopic quantum states of light.

September 1, 2023

Air Force Research Laboratory, Asian Office of Aerospace Research and Development, Tokyo,Japan

The image shows noise level of anti-squeezed (blue) and squeezed light (red) vs. a loss of fog.

More than half a century has passed since the birth of quantum signal detection theory, which is the cornerstone of modern quantum communication theory. Quantum stream cipher, the quantum-noise-based direct encryption scheme for optical communications at the center of our research, is based on the foundations of quantum communication theory. For quantum cryptography to progress from a theoretical possibility to a more realistic technology, experimental and theoretical research must be complementary.

We have reported several experimental and theoretical studies on the quantum stream cipher connecting two points via optical fibers and also fabricated a prototype based on them. To enhance the usability of a quantum stream cipher, free-space optical communications must be explored in addition to point-to-point optical communications connected by optical fibers. In the case of free-space optical communications, various environmental changes caused by the weather affect the communication channel. Therefore, quantum communications, including cryptographic applications, must be considered from experimental and theoretical perspectives under various harsh weather conditions such as fog, rain, snow, and turbulence.

Our project aims to explore the possibility of all-weather secure quantum communication using macroscopic quantum states of light. The goals of this project are the (a) experimental elucidation and mathematical modeling of the propagation characteristics of macroscopic quantum states of light owing to atmospheric turbulence and (b) basic research on quantum receivers for cryptographic applications in harsh environments.

We built a simulation chamber for a uniform and non-uniform fog and experimentally observed the propagation characteristics of visible, near-infrared, and single-mode squeezed light, respectively. The experiments confirmed that the effect of fog appeared mainly in the form of energy loss. Future work is required to simulate other environments, not limited to fog, and experiment with entangled light, such as two-mode squeezed light.

We devised an optical processing method that simultaneously performed decryption of quantum stream cipher and homodyne detection. The proposed method that manipulates the phase of local light can perform the same decryption function as the conventional one. Furthermore, since the cryptographic signal is directly detected without additional attenuation, it is expected to simultaneously achieve decryption and homodyne detection in the shot noise limit.

We conducted a proof-of-concept experiment of the proposed decryption method (unpublished). In addition, theoretical analysis was also performed. Therefore, we confirmed that the experiments and theory were consistent. However, future work is needed to improve the experimental accuracy and closely align experiments and theory by conducting theoretical analyses that include more practical conditions.

We developed a simple method for numerically determining the error probability characteristics of homodyne receivers and optimal quantum receivers when the model of a turbulent communication channel is given by the probability distribution of the transmission coefficient. Using the model in reference [Semenov & Vogel], we investigated the error probability characteristics of the homodyne receivers and the optimal quantum receiver under certain turbulent conditions. Future issues include treating various free-space communication channels, designing a system that leverages the robustness of the homodyne receiver confirmed in this study, and the realization problem of an optimal quantum receiver for the harsh environments encountered in free-space optical communications.

This work was performed by Souma Masaki for the Air Force Research Laboratory Asian Office of Aerospace Research and Development. For more information, download the Technical Support Package (free white paper) below. ARFL-20230054