Free-Space Quantum Communications in Harsh Environments

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

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

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.