# Wavelength Selection for Satellite Quantum Key Distribution
## 4 QKD Performance
### Adaptive Optics  
Adaptive optics (AO) systems have the potential to significantly improve performance for satellite QKD by overcoming atmospheric turbulence. [PERSON] et al. performed a field experiment showing free-space QKD over 53 km free-space link across Qinghai Lake in daylight [61]. A single-mode fibre was used to spatially filter optical noise, along with a fast steering mirror, a deformable mirror, and an optical tracking system with 300 Hz feedback frequency. In [58], a satellite QKD analysis combined AO and a single-mode optical fibre which reduced background noise and improved fibre coupling efficiency. A 200-Hz bandwidth closed-loop AO system was shown to be sufficient to facilitate daytime QKD, even with the high slew rates associated with 400 and 800 km LEO orbits. [PERSON] et al. followed this with a terrestrial field-site validation of a 2 kHz frame rate, and a 130 Hz closed-loop bandwidth AO system across a 1.6 km horizontal path in daylight under carefully tuned 700 km downlink conditions [23]. These studies clearly show that adaptive optics combined with suitable spatial, spectral and temporal filtering improve the performance of satellite and free-space QKD. However, several challenges remain in scaling these experimental findings to operational satellite QKD systems under realistic environmental and hardware constraints. AO systems are complex, containing specialized modules requiring high stability to operate in remote locations and often harsh environments with limited space. Future miniaturization of detectors and optical components will improve system performance and aid the transition to operational satellite QKD systems. AO systems utilize a deformable mirror to correct turbulence-induced wavefront aberrations and have been shown to significantly improve the performance of QKD systems operating in a turbulent channel [22; 60; 62]. Adaptive optics is used to guide a beam of photons from the transmitter to the receiver, regardless of the type of QKD protocol being used. If the transmitter is on the ground, a guide star can be used to pre-compensate for the effects of atmospheric turbulence and align the QKD laser source on the ground with the receiver on the satellite. Conversely, if the transmitter is on the satellite, a beacon laser can be used to correct for distortions in the wavefront due to atmospheric turbulence and align the QKD laser source on the satellite with the receiver on the ground. The optical ground station Cassegrain reflecting telescope consists of an AO module and a quantum receiver module. Beacon and signal (at slightly different wavelengths) are sent from the satellite sources to the telescope and directed to the separate modules via a dichroic mirror [23; 58]. A Shack-Hartmann wavefront sensor (SHWFS) measures higher-order wavefront errors. A fast steering mirror (FSM) compensates for a low-order wavefront tilt while a deformable mirror (DM) compensates for higher-order wavefront aberrations. In a closed-loop AO system, dynamic feedback control reduces residual wavefront errors. Here, we adopt a 200 Hz closed-loop bandwidth AO system which yields an effective \(r_{0}\approx 50\) cm [58]. \(r_{0}\) can be varied to investigate how performance is impacted by turbulence-induced phase changes and how an adaptive optics system can improve the secret key rate. There are several practical challenges faced when integrating AO in space-borne systems. Two essential components of AO systems are the deformable mirror (DM) and the Shack-Hartmann wavefront sensor (SHWS). DeMi is a 6U CubeSat mission that characterized the on-orbit performance of a 140-actuator microelectromechanical system (MEMS) DM with an image front sensor and a SHWFS [63]. MEMS DMs are a promising technology for space-based adaptive optics because of their high actuator density, and low size, weight and power (SWAP) requirements.