LEO-to-ground polarization measurements aiming for space QKD using Small Optical TrAnsponder (SOTA).

Quantum communication, and more specifically Quantum Key Distribution (QKD), enables the transmission of information in a theoretically secure way, guaranteed by the laws of quantum physics. Although fiber-based QKD has been readily available since several years ago, a global quantum communication network will require the development of space links, which remains to be demonstrated. NICT launched a LEO satellite in 2014 carrying a lasercom terminal (SOTA), designed for in-orbit technological demonstrations. In this paper, we present the results of the campaign to measure the polarization characteristics of the SOTA laser sources after propagating from LEO to ground. The most-widely used property for encoding information in free-space QKD is the polarization, and especially the linear polarization. Therefore, studying its behavior in a realistic link is a fundamental step for proving the feasibility of space quantum communications. The results of the polarization preservation of two highly-polarized lasers are presented here, including the first-time measurement of a linearly-polarized source at λ = 976 nm and a circularly-polarized source at λ = 1549 nm from space using a realistic QKD-like receiver, installed in the Optical Ground Station at the NICT Headquarters, in Tokyo, Japan.

Polarization measurements through space-to-ground atmospheric propagation paths by using a highly polarized laser source in space.

The polarization characteristics of an artificial laser source in space were measured through space-to-ground atmospheric transmission paths. An existing Japanese laser communication satellite and optical ground station were used to measure Stokes parameters and the degree of polarization of the laser beam transmitted from the satellite. As a result, the polarization was preserved within an rms error of 1.6 degrees, and the degree of polarization was 99.4+/-4.4% through the space-to-ground atmosphere. These results contribute to the link estimation for quantum key distribution via space and provide the potential for enhancements in quantum cryptography worldwide in the future.

Laser reflector arrangement on H2A-lRE satellite for monitoring spin rate and optical degradation.

We devised a unique array of cube-corner reflectors on a spherical satellite so that laser ranging could be used to detect the spin rate and the optical degradation. On the H2A-LRE satellite, there are reflectors made of synthetic silica and ones made of BK7 glass. We report that the degradation of the BK7 reflectors, as well as the spin rate of the satellite, can be monitored through a spectral analysis of laser-ranging data. The center-of-mass correction of the Laser Ranging Equipment satellite was calculated at 210 +/- 10 mm for all possible systems and throughout the degradation process of the BK7 reflectors.