Quantum cybersecurity in the real world
While the scientific motivation is clear, interpreting Bennet and Brassard’s work in terms of cybersecurity is a subtle task. Mathematically, quantum cryptography has an infinite added-value compared to classical methods. In the real world, things get more complicated. Firstly, the security of communication systems relies not only on confidentiality, but also on authentication of its participants. Quantum cryptography only applies to the first one, and designing a fully unconditionally secure cryptosystem using quantum key distribution is a difficult task. Classical authentication methods with unconditional security exist, but do not scale well in practice. This directly impacts both the design of quantum networks and the trust model of the security infrastructure.
Hardware constraints are an even bigger obstacle to the development of quantum communication networks. Encoding information at the quantum scale requires to process light at the single photon level. This cannot be done using standard telecom hardware, which immediately implies large hardware costs. In addition, when travelling in optical fiber, most photons are lost. This greatly limits the distance between two QKD nodes.
Even worse, each of these two hardware constraints push the design of quantum networks toward two opposite paradigms. On the one hand, the limitation on the distance QKD suggests deploying quantum key distribution at a small-scale. But on the other hand, the hardware cost reserves QKD to critical sites, which are in general not close to each other.
The best example of resolution of these constraints is the Chinese quantum communication network, which spans over more than 2000km and more than 30 nodes. The Chinese approach to the distance problem is to include trusted nodes to route keys between distant nodes. In this approach, the trust in trusted nodes is not cryptographic, but relies on a security guard preventing any attempt to compromise the node physically.
The Chinese QKD infrastructure
While the Chinese network is a perfectly legitimate solution to the problem, it is to a large extent irreproducible. It requires a very specific economic set-up both for the initial investment and for running up the network. Finding innovative ways to overcome the hardware barriers seems essential for a wide development of quantum networks. Increasing the distance and decreasing the costs are the two options to consider new network topologies. They are investigated by many startups working on integrated photonics, space communications or quantum information processing hardware.