A research team led by York University – home to the UK’s quantum communications “hub” – has demonstrated that ultra-secure quantum encryption protocols should work over a 224-kilometer commercial-grade submarine cable linking the UK and Ireland.
In a pilot project conducted in July, York professor Marco Lucamarini and colleagues tested key elements of continuous-variable quantum key distribution (CV-QKD) over a low-loss fiber deployed by the network operator euNetworks.
Known as “Rockabill”, the fiber-optic link runs between Portrane in Ireland and Southport in the UK and is said to offer “remarkably low” average attenuation, with no need for amplification or any repeaters.
“Until now, no quantum link has ever been established between the two countries, nor on a span stretching this length on a subsea fiber-optic cable,” stated the team in a release announcing the results.
Sensitive detectors crucial
While quantum encryption has been possible for many years it has typically been limited to relatively short distances or specialty fiber links. This is because it relies on the physical properties of individual photons, and the chances of photon loss due to attenuation or scattering increase dramatically over longer distances.
One way around this is to use “trusted nodes” at stages in the fiber-optic link, but the euNetworks sub-sea link does not feature any such nodes.
The research team said that the experiments conducted on-site resulted in the successful transportation of both single and entangled photons, as well as in the successful measurement of optical phase – the key physical attribute used to secure twin-field and CV-QKD protocols.
Results from the project, which is funded via the UK National Quantum Technologies Programme, were also presented earlier this week in Amsterdam, during the NATO Symposium on Quantum Technology for Defence and Security.
The research team says that the success of the initial tests was largely due to some highly sensitive detectors deployed at the Southport terminal of the optical link, which are able to reduce environmental noise levels.
CV-QKD also has an advantage over traditional QKD links, with the latter requiring expensive low-temperature single-photon avalanche detectors (SPAD) to operate.