# Vulnerability of Satellite Quantum Key Distribution to Disruption from Ground-Based Lasers
## 5 Conclusions  
We have shown that modern satellite-mediated QKD can be disrupted by noise caused by excess photons transmitted from a ground based laser terminal and scattered from the QKD satellite. The laser terminal needs a continuous launch power of around 1 kW to achieve complete disruption of a system based on that demonstrated by the _Micius_ satellite. Lasers of this class are available commercially, and a malicious party could construct a simple laser terminal capable of causing significant disruption of QKD links at distances up to 1000 km for less than $200,000 USD. This means that such disruption is a significant security threat that should be taken into account in the design and use of future QKD satellite systems. We developed a semi-empirical scattering model for the QKD satellite and performed orbital dynamics calculations to estimate the excess photon rate from a belligent laserterminal reaching a QKD ground station up to 1000 km from the laser terminal. This photon rate is used to determine the additional QBER caused by the excess photons, which introduce noise into the QKD receiver. We identify a number of countermeasures and design considerations future satellite QKD systems can implement to reduce the impact of this form of disruption. Likely future developments including moving to higher orbits and improved spectral and spatial filtering required to enable satellite QKD links to operate in daytime will improve the resilience of these links to disruption from remote lasers. However, all of these countermeasures simply require the laser terminal to increase its power on target to restore the same level of disruption. We also discuss how this form of disruption could impact future satellite QKD technology. In particular, where the satellite includes an on board QKD receiver (the satellite supports a quantum memory or quantum repeater) will be even more vulnerable to disruption from a ground-based laser. Other potential methods to disrupt satellite-mediated QKD link include mounting the laser terminal to an aircraft, scattering of noise photons from the atmosphere, and the generation and dispersal of scattering aerosols. Our analysis shows these disruption methods pose a credible threat to satellite-mediated QKD in the future, and should be considered in the development and effective use of QKD satellite for high-value communications for areas such as finance, government operations, and defence. Additional analysis is required to determine the effects of this form of disruption on QKD protocols other than decoy-state BB84. The scattering and attenuation models used to predict the excess photon rate in this analysis could be verified by measuring the detected photon rate from bistatic satellite laser ranging experiments [23]. Conceptualization, D.R.G. and T.W. ; methodology, D.R.G. and T.W. ; software, S.W. and T.W. ; validation, D.R.G., S.W. and T.W. ; formal analysis, D.R.G., S.W. and T.W. ; investigation, D.R.G., S.W. and T.W. ; writing--original draft preparation, D.R.G. ; writing--review and editing, S.W. and T.W. ; project administration, D.R.G. ; funding acquisition, D.R.G. and T.W. All authors have read and agreed to the published version of the manuscript. This research was funded by the Australian Research Council Centre of Excellence for Engineered Quantum Systems (EQUS, project ID CE170100009) and by the Australian Army Quantum Technology Challenge. [PERSON] is supported by a Forrest Research Foundation Fellowship. [PERSON] was supported by the Transitional Research Laboratory of EQUS for this project. Data Availability Statement: Data supporting this study can be obtained from the authors upon reasonable request. The authors would like to thank [PERSON], [PERSON], and [PERSON] for their assistance and advice, and the Translational Research Laboratory of EQUS for their financial support. The authors declare no conflict of interest.