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Free-Space Quantum Communications in Harsh Environments

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

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

Topics:
Cryptography Data acquisition and handling Fiber Optics Harshness Lasers & Laser Systems Optical Components Optics Optics Photonics Research and development Water
This Brief includes a Technical Support Package (TSP).
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Free-Space Quantum Communications in Harsh Environments

(reference ARFL-20230054) is currently available for download from the TSP library.

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    Aerospace & Defense Technology Magazine

    This article first appeared in the September, 2023 issue of Aerospace & Defense Technology Magazine.

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    Overview

    The document titled "Control of Quantum Noise for Optical Sensing and Communications" is a final technical report authored by Masaki Souma from Tamagawa University, detailing a research project supported by the Air Force Office of Scientific Research (AFOSR) through the Asian Office of Aerospace Research and Development (AOARD). The project, conducted from September 25, 2020, to September 24, 2022, aims to explore the potential for all-weather secure quantum communication using macroscopic quantum states of light.

    The research is structured around two primary goals: (a) experimental investigations and mathematical modeling of the propagation characteristics of macroscopic quantum states of light affected by atmospheric turbulence, and (b) foundational research on quantum receivers designed for cryptographic applications in harsh environmental conditions. The project emphasizes the importance of understanding how various atmospheric disturbances, such as fog, rain, and turbulence, impact the transmission of quantum signals.

    To achieve these goals, the team conducted laboratory experiments simulating fog to study the propagation characteristics of visible laser light, near-infrared laser light, and single-mode squeezed light. The findings indicated that uniform fog primarily causes energy attenuation due to Mie scattering, which is critical for understanding signal degradation in real-world conditions.

    Additionally, the research includes discussions on an optical pre-processing method aimed at enhancing the reception sensitivity of optical homodyne detection. The team performed proof-of-principle experiments for a modified homodyne detection technique and conducted theoretical analyses to support their findings. They also developed a numerical analysis method to evaluate the error probability characteristics of optical receivers operating in turbulent free-space communication channels, where the transmission coefficient varies probabilistically.

    The report highlights several publications and significant collaborations resulting from the project, including peer-reviewed papers and presentations at conferences. Notable works include investigations into light wave propagation in atmospheric disturbances and studies on quantum communication through turbulent atmospheres.

    Overall, this research contributes to the advancement of quantum communication technologies, particularly in developing robust systems capable of functioning under adverse weather conditions, thereby enhancing the feasibility of secure quantum communications in practical applications. The findings underscore the necessity for continued experimental and theoretical work to bridge the gap between theoretical possibilities and real-world implementations in quantum cryptography.

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