# Vulnerability of Satellite Quantum Key Distribution to Disruption from Ground-Based Lasers
## 1 Introduction  
Rapid developments in quantum computing threaten the forward security of public key cryptography that currently underpins secure communication and trade. Quantum key distribution (QKD) [1] uses the laws of quantum mechanics to provide a guaranteed information-secure cryptographic key exchange between two parties. As QKD technology matures to enable practical key rates over long distances, it will become of extreme importance to areas such as finance, government operations, and defence.  
A great deal of effort is being expended in developing practical QKD technologies around the world, and rapid progress is being made on many fronts [2]. However, due to link attenuation and the impossibility of noiselessly amplifying quantum states, QKD via terrestrial optical fibre links is currently limited to a few hundred kilometres [3, 4] and extending this limit is extremely challenging [5]. Quantum repeaters, that extend the reach of QKD systems via entanglement swapping [6], are being developed, but these technologies have not yet matured to a practical level.  
Satellite-mediated QKD has been demonstrated as the most promising method for creating practical long-distance (on the order of thousands of kilometres) QKD links [7, 8]. Optical losses on the free-space link are dominated by geometric factors due to the spreading of the optical beam, while fibre-based QKD systems are limited by absorption, scattering, and quantum state decoherence in the glass fibre. The _Micius_ satellite [7], launched in 2016, achieved an average quantum key rate of \(1.1\) kb/s over a maximum distance of1200 km, with a quantum channel efficiency 20 orders of magnitude greater than for an optical fibre link of the same maximum length. Advances in other areas of QKD research and development, such as practical quantum repeaters and quantum memories, will be directly applicable to satellite-mediated QKD links, meaning satellite QKD is likely to remain at the forefront of practical QKD link deployment for many years.  
While the quantum security of a QKD link between a satellite and ground station is beyond doubt [8], the high-value nature of communications secured via QKD means that attempts to disrupt (or 'jam') the transmission of the quantum key by malicious parties are a possibility. In this paper, we show that ground-based lasers of moderate power (on the order of 1 kW depending on the link architecture and geometry) reflecting off the satellite add sufficient excess optical noise to completely disrupt QKD transmissions from low Earth orbit satellites. Today, laser amplifiers on the order of a few kW are available commercial off-the-shelf. Along with the necessary optics and tracking mount, a system able to target a satellite and disrupt its QKD transmission can be assembled for under $200,000 USD, meaning that the means to disrupt satellite QKD are available to even small groups with modest resources. This poses a serious threat to the use of satellite-mediated QKD in the future, and we identify measures that future satellite QKD systems could employ to reduce their susceptibility to this type of disruption.