# Wavelength Selection for Satellite Quantum Key Distribution
## 1 Introduction
This results from the use of a multiphoton source with high laser repetition rates (e.g., 50 MHz), many more degrees of freedom for quantum state encoding, homodyne or heterodyne detectors with high efficiency (0.95) and compatibility with standard telecommunications and lower coupling losses. CVQKD could potentially also operate in strong stray light and in daytime as it filters out background radiation. Substantial challenges exist when considering the practical feasibility of satellite CVQKD, for example, setup stabilization, reduction of noise and reduction of loss. Another major challenge is sharing the local oscillator between Alice (satellite transmitter) and Bob (ground receiver), so that they have the same reference frame to encode and measure the phase information [34]. One reason why polarization DVQKD protocols are used successfully in satellite QKD is that the polarization of the laser beam is affected less than the phase as it propagates through the atmosphere. [PERSON] et al. provide a comparison of CV and DV protocols for satellite-Earth FSOC in terms of maturity, cost, robustness to space operations, and other operational parameters [15]. It should be noted that, to date, nearly all satellite-based demonstrations of QKD have been DV-based [13; 19]. Therefore, we have based our work on DV-QKD using the decoy-state BB84 protocol. The Micius satellite has been used to demonstrate QKD, entanglement distribution and quantum teleportation. Moreover, it has paved the way for developing global quantum communication networks [35]. In [36], an analysis was performed on data from the Micius satellite trusted-node downlink using BB84 weak coherent decoy states. The satellite to ground transmission window was limited to 440 s for a zenith pass, and the effects of background light, source quality, and overpass geometries were estimated during the modelling of the satellite QKD key generation capacity. Background noise was found to be the limiting factor and that developing methods to suppress background light are a priority. In [37], an upper bound to the maximum finite secret key length (SKL) generation capacity was established in order to inform system engineering development, operation and deployment. A demonstration of a space-ground QKD network included a compact terminal on the Tiangong-2 Space Lab connected to four ground stations, and showedthat medium-inclination orbit and Sun-synchronous orbits are suitable for future quantum satellite constellations [38]. Recent advances in metasurfaces, nanophotonic systems and photonic integrated circuits pave the way for compact devices in secure key encryption and quantum communication [39; 40]. The focus of this work is on optimizing satellite QKD performance for a variety of atmospheric conditions using a range of techniques to mitigate losses. Regions of the electromagnetic spectrum that produce the highest secret key rates \(R\) for satellite QKD downlink channels operating in daylight under realistic and adverse atmospheric conditions are identified. Suitable filtering techniques, adaptive optics, and wavelengths selection are combined. Moreover, secret key rates, \(R\), are evaluated for a range of physical component sizes. This paper is structured as follows: In Section 2, transmittance and spectral radiance are calculated for different regions of the electromagnetic spectrum. Section 3 outlines a channel model accounting for free-space diffraction, turbulence and background noise. Section 4, which focuses on QKD performance, describes the calculations of the secret key rate, \(R\), using the decoy-state BB84 protocol [41], combined with adaptive optics, spectral and temporal filtering. In Section 5, the simulation results are analysed to identify optimum wavelength regions for satellite QKD. For practical QKD, there is often limits imposed on receiver and transmitter sizes so that the best wavelengths for a range of component sizes are determined.