Entangled photons are distributed over vast distances using a satellite-to-ground link.

Photon-counting detectors can increase the range of underwater optical wireless communication (UWOC) by enhancing the detection sensitivity and have thus become an active research area in recent years. However, system performance still needs to be improved in the reported underwater photon-counting communication (UPCC) experiments. In this study, a real-time, high-speed UPCC system was designed and experimentally validated based on a single-photon avalanche diode (SPAD). A reliable symbol synchronization method based on pulse-position modulation (PPM) was developed. This method achieved slot synchronization by using a narrow-pulse-width laser and a simple matched filter and realizes frame synchronization through an improved synchronization sequence. In addition, channel errors were corrected by serially concatenated convolutionally coded PPM (SCPPM), and digital signals were processed in real time based on field-programmable gate arrays (FPGAs). Finally, desktop communication experiments were completed at a slot frequency of 25 MHz and a communication rate of 6.21 Mbps. The system exhibited a bit error rate (BER) of less than 10 ^–7 , a received optical power of only −84.3 dBm, and an efficiency of 1.35 photons per bit.

The features of unmanned aerial vehicles (UAVs), such as strong manoeuvrability, small size, and light weight, make them considered to participate in free space optical (FSO) quantum communication, becoming an important part of the Space-Air-Ground Integrated Network. However, research reports on small-sized drones based long-range optical quantum link are few. A major challenge is that traditional opto-mechanical structure is difficult to adapt to small UAV platforms. Here, a lower size, weight, and power (SWaP) acquisition and tracking system prototype is developed, and its greatest advantage is that the system can be easily integrated into small flying platforms and quickly build a long-distance quantum communication link. The prototype weighs less than 8 kg. A small-sized two-axis mirror mechanism is designed for wide-range acquisition and tracking. Moreover, the system can automatically point to ground station. The SWaP acquisition and tracking system can support quantum key distribution (QKD) link and general FSO link by drones. In the 7 km air-to-ground demonstration, the acquisition and tracking system prototype is tested in practice, which is suspended under the small six-rotor UAV platform, and a special passive vibration isolation unit is added in between. A ten-minute continuous and stable beacon tracking proves that the system can quickly and automatically align.

In this study, an active microvibration isolation system is developed for precision space payload. Vibrational environment affects the performance and reliability of measuring instruments. To improve the measurement accuracy of the precision space payload, an active vibration isolation system based on eight vibration isolation modules, which are applied for microvibration isolation on the satellite, is designed. A vibration suppression control strategy for multiple degrees of freedom is studied. A hybrid control method involving a feedback and a feedforward controller based on a nonlinear tracking differentiator and an nth-order weak integrator, respectively, was adopted to optimize the suppression effect of microvibration. As a result, the microvibration of the order of mg can be reduced to the order of μg through the active control of modules. Research experiment results show that the root cumulative power spectral density of the systemic sensitive frequency band in the range of 0.5–200 Hz, i.e., microvibration frequency band in the optical reference cavity, has been reduced to in the order of μg in three directions, which satisfied the requirements of aerospace engineering.

Satellite angular micro-vibration has an important impact on the efficiency of space quantum communication links. We measured the micro-vibrations on the Micius satellite in orbit using a high-precision optical sensor mounted on the satellite and analyzed the power spectral density. We designed a compound axis acquisition, tracking, and pointing (ATP) system based on a two-axis turntable and tested its suppressive effect on the micro-vibration through in-orbit experiments. The tracking error caused by the angular micro-vibration was found to be 9.3 µrad, with the energy concentrated primarily in the frequencies below 30 Hz; after suppression by the ATP system, the error was 0.47 µrad.

The optical reference cavity in an ultrastable laser is sensitive to vibrations; the microvibrations in a space platform affect the accuracy and stability of such lasers. In this study, an active vibration isolation controller is proposed to reduce the effect of vibrations on variations in the cavity length and improve the frequency stability of ultrastable lasers. Based on the decentralized control strategy, we designed a state-differential feedback controller with a linear quadratic regulator (LQR) and added a disturbance observer (DOB) to estimate the source noise. Experiments were conducted using an active vibration isolation system; the results verified the feasibility and performance of the designed controller. The accelerations along the axis (Z-, X-, Y-) directions were suppressed in the low-frequency band within 200 Hz, and the root-cumulative power spectral densities (PSDs) declined to 1.17 × 10⁻⁵, 7.16 × 10⁻⁶, and 8.76 × 10⁻⁶ g. This comprehensive vibration met the requirements of an ultrastable laser.