A recent paper published in PRX Quantum gives a boost to quantum secure communications via satellite. The team – led by Leeds researchers and involving Quantum Communications Hub colleagues from the Universities of Bristol and York, alongside international collaborators from Germany, Singapore and the Swiss company ID Quantique – focused on the limitations that potential eavesdroppers may face in practice in collecting and transmitting quantum signals in satellite-to-ground links. The team conclude that satellite-based quantum systems could achieve higher secure key rates than was previously expected.
Quantum key distribution (QKD) is a mature quantum technology, which protects communications and other transactions through the quantum-secure distribution of cryptographic keys used to encrypt and decrypt data. QKD by satellite addresses the distance limitations inherent to this technology over terrestrial optical fibre, which result from signal loss and are a barrier to quantum-secure communications on continental, intercontinental and global scales.
Conventionally, quantum communications security protocols characterize signal transmissions between two communicating parties (described as “Alice” and “Bob”) while considering the potential disruptive effect of an unwanted eavesdropper (“Eve”). Lead researcher, Professor Razavi from the University of Leeds commented: “One of the intriguing aspects of security proofs for QKD systems is that they rely on minimal assumptions on Eve to guarantee security in most stringent conditions. As such, in QKD jargon, Eve is considered to be in full control of the channel that connects Alice and Bob.”
In the real world, Eve could hardly be this effective at intercepting a satellite link. Orbiting objects could be spotted by LIDAR surveillance if they got too big, and the area around the telescope on the ground could be searched and protected. Razavi noted “there has been little attention to classical characteristics, such as size, weight, or energy consumption, of Eve’s apparatus. Our work analyses the security of QKD systems without restricting Eve’s quantum processing power, but rather by assuming some size limitations on flying objects that Eve may need to employ in a line-of-sight link.” This implies limits on how much of the signal between the satellite and ground station Eve could receive. The result is that there exists so-termed “bypass transmission channels”, where at least part of the signal between Alice and Bob could bypass Eve.
Collaborator Alexander Ling, an Associate Professor at the National University of Singapore, first spoke to the UK team about these ideas when he was planning for the 2019 launch of SpooQy-1 – a nanosatellite his group built to test technology for quantum communications. “I was looking at how a satellite orbits the ground station and how the signal comes to ground, and I realised that an eavesdropper cannot simply do whatever they wish. They are constrained by satellite orbital physics and diffraction of beams.”
The research presented in this paper considers such realistic restricted eavesdropping scenarios. The team’s analysis of achievable QKD key rates in the presence of bypass channels highlights considerably improved performance for certain security protocols. Any performance improvements could contribute to commercial exploitation of satellite-based quantum communications technology and global-scale deployment of QKD systems.
The results open up new possibilities for future research directions in QKD security, especially when delivered via satellites, or when eavesdropping is naturally restricted. “Usually when we try to make QKD practical we find eavesdropping mechanisms we need to guard against, this time the real world scenario helps us” said Professor Ling. “This results in new security scenarios that we investigate in our work, and creates many others that are yet to be explored” Razavi added.
Ghalaii M, Bahrani S, Liorni C, Grasselli F, Kampermann H, Wooltorton L, Kumar R, Pirandola S, Spiller TP, Ling A, Huttner B, and Razavi M. Satellite-Based Quantum Key Distribution in the Presence of Bypass Channels. PRX Quantum 4, 040320 – Published 1 November 2023