Continuous-variable quantum key distribution (CV-QKD) systems in China have shattered distance and speed records when implemented on commercial fibre networks. Operating in two cities, the systems distributed cryptography keys up to 50 km (three times further than the previous record) and with secret-key transmission rates more than 100 times faster than previous systems.
Quantum key distribution (QKD) involves two people (usually called Alice and Bob) sharing a secret key that they can use to encode and decode messages. The key is encoded into a string of quantum entities (usually photons), so that any eavesdropper (Eve) attempting to copy the key as it passes from Alice to Bob reveals her presence by virtue of the laws of quantum mechanics. These laws dictate that the act of measuring affects the system being measured.
In CV-QKD, the quantum key is encoded in the phases of weak coherent laser pulses – the phases being continuous variables. CV-QKD offers several benefits over early implementations of QKD, which used single photons. CV-QKD can be implemented using standard optical telecoms components, for example, and can also operate at higher secret-key transmission rates on city-sized (metropolitan) networks.
Dark fibre links
In this latest demonstration, Hung Guo, Yichen Zhang, Song Yu and colleagues at Peking University and Beijing University of Posts and Telecommunications operated CV-QKD systems in Xi’an and Guangzhou using commercial fibre links. In both cases the links were “dark”, which means that they carried no other telecoms traffic during the tests.
In Xi’an the team transmitted secret keys a distance of 30 km, whereas nearly 50 km transmission was achieved in Guangzhou – smashing the previous record of just over 17 km for CV-QKD over commercial fibre. Although quantum cryptography has previously been achieved over distances of hundreds of kilometres, these demonstrations were done on dedicated fibres rather than on commercially-deployed telecoms links.
A secret key transmission rate of about 6 kilobits per second (kps) was achieved, much faster than the previous record of about 0.3 kps.
Encrypted quantum keys sent a record-breaking 421 km in optical fibre
The ability to operate CV-QKD on commercial networks will be crucial to the widespread adoption of the technology – which Guo says could be used to protect banking information, email messages and passwords. However, quantum information is very fragile and this makes deploying CV-QKD in a commercial network is a significant challenge.
“Deployed commercial dark fibres are inevitably subject to much stronger perturbations from changing environmental conditions and physical stress,” explain Zhang and Yu, adding, “This in turn causes severe disturbances of the transmitted quantum states”. They add that the networks, “also suffer from higher losses due to splices, sharp bends and inter-fibre couplings”.
The research is described in Quantum Science and Technology.