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

Abstract

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.

LEO-to-ground optical communications using SOTA (Small Optical TrAnsponder) – Payload verification results and experiments on space quantum communications

“…They are non-orthogonally linearly polarized laser sources, since the most important QKD protocols are based on polarization-encoded photons. In the first phase of this experiment, a characterization of the polarization behavior after the atmospheric propagation was carried out [32]. For this, a 1.5-m Cassegrain telescope was used to gather the signal from SOTA and couple it to a polarimeter ( Fig.…”

Satellite-to-ground quantum-limited communication using a 50-kg-class microsatellite

“…Data-intensive satellite sensors mounted in such a constellation produce a large amount of information to be transmitted to the ground in a short time, which requires high capacity communications.However, conventional satellite communications based on microwave frequency bands will struggle to provide the needed capacity because these bands are already congested and severely regulated, and hence the frequency licensing process is lengthy. In the last decade, laser communication (lasercom) has evolved as a promising alternative for high-capacity data links from space [1][2][3][4][5][6][7][8][9][10][11], overcoming microwave communication in several key aspects, such as much higher data rates, being able to use an unregulated spectrum, ultra-low inter-channel interference, smaller and lighter terminals, and power-efficient transmission. In fact, the feasibility of satellite lasercom has been demonstrated by many space missions so far.…”

Progress in satellite quantum key distribution

“…Polarization preserved within system rms error of 28 mrad. Takenaka et al [90,91] LEO-to ground polarization and quantum limited measurements from a small optical transponder (SOTA). SOCRATES 48 kg microsatellite.…”

“…For the analysers to be effective, systems must be in place to allow dynamic polarization variations -most particularly the relative roll orientation of the satellite and ground station -to be understood and compensated, so that the reference frame of the polarization bases is preserved. This is a complex task for non-GEO orbits as the satellites are moving across the sky throughout the operation, but it can be achieved without the requirement for a feedback loop between space and ground [87][88][89][90]. In all tests, atmospheric depolarization has been found to be minimal.…”