# Wavelength Selection for Satellite Quantum Key Distribution
## 6 Conclusions  
Developing a global quantum communication network poses several challenges, such as distance limitations of QKD over fibre optic networks. Satellite QKD is a promising solution that has matured from a theoretical concept to real-world demonstrations with the Micius satellite mission. Here, we determine optical wavelengths for QKD in the satellite downlink channel while operating through realistic daytime atmospheric conditions. The optimal wavelengths for QKD in the satellite downlink channel were determined for realistic daytime atmospheric conditions. A channel model was developed using the BB84 decoy-state QKD protocol as a performance metric. The secret key rate was determined for a wide range of wavelengths while considering operation in a broad range of atmospheric, geometric, and geographical conditions. Two base scenarios were considered, the clear skies of a typical summer's day in Tucson, Arizona, to represent an ideal location for satellite communication and the overcast climate of an Irish winter, to represent a non-ideal location. On a relatively clear winter's day with light cloud cover, Waterford is capable of yielding high secret key rates due to low solar radiance. However, Irish skies are often covered by clouds. Therefore, when more challenging atmospheric conditions are introduced, satellite communications become less practical.  
Figure 7: Transmittance and spectral radiance for a ground receiver pointed at the zenith at noon on the winter solstice in Waterford, through (a) 5 km visibility haze, (c) 500 m visibility radiative fog, and (e) drizzle rain clouds. Parts (b,d,f) are 3D surface plots of secret key rates versus wavelength and receiver diameters in adverse weather conditions.  
Visible and NIR wavelengths benefit from a small beam spot size at the receiver, resulting in low diffraction losses and high transmittance. However, high solar radiance means that small receivers must be used to filter out excess noise photons, which reduces the secret key rate. The importance of the visible and NIR regions is that they (i) enable high secret key rates without requiring large receivers and (ii) can implement filtering techniques when operating through adverse weather conditions. The SWIR window has the highest transmittance across the whole spectrum and the lowest spectral radiance. However, it suffers from beam spread loss and requires large receivers to produce high secret key rate performance.  
Future work could focus on an uplink scenario. Our analysis of the channel could be extended to include acquisition, tracking, and pointing (ATP), in addition to considering more angles. Furthermore, an analysis of potential attack scenarios and the impact of eavesdropping on the channel should be considered. Additionally, the altitude of the satellite is an important variable and requires further investigation to determine how it affects wavelength selection. Finally, a comparison with a CVQKD-based protocol, especially at longer wavelengths, would extend this work.  
Conceptualization, S.H., J.H. and D.K.; Methodology, S.H., N.B. and D.K.; Software, S.H.; Formal analysis, S.H.; Writing--original draft, S.H.; Writing--review and editing, J.H. and D.K.; Supervision, D.K. All authors have read and agreed to the published version of the manuscript.  
This publication has emanated from research conducted with the financial support of _Taighde Eiraun--Research Ireland_ under Grant number 13/RC/2077_P2 at _CONNECT: the Research Ireland Centre for Future Networks_ and under Grant number 21/RC/10294 P2.  
Not applicable.  
Data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.  
The authors declare no conflicts of interest.