Ben Amies-King

The future quantum internet will leverage existing communication infrastructures, including deployed optical fibre networks, to enable novel applications that outperform current information technology. In this scenario, we perform a feasibility study of quantum communications over an industrial 224 km submarine optical fibre link deployed between Southport in the United Kingdom (UK) and Portrane in the Republic of Ireland (IE). With a characterisation of phase drift, polarisation stability and the arrival time of entangled photons, we demonstrate the suitability of the link to enable international UK–IE quantum communications for the first time.

Development of Quantum Key Distribution (QKD) over long horizontal distances has provided both potential use cases for horizontal links within future quantum networks and testbeds to test protocols for satellite QKD. However, the majority of these implementations have used the polarisation of light as encoding scheme, with little work performed on phase-encoded schemes. Given the advantages that recent phase-based protocols such as ‘twin-field’ (TF) QKD have within fibre, it is possible the same distance-rate benefits can be found with free-space phase-based protocols.

Quantum key distribution (QKD) enables secure communications against an adversary with unbounded classical and quantum computing capability. Since the original BB84 protocol was proposed, various other protocols have been developed with specific hardware requirements on the quantum layer. However, in general time synchronisation and a classical communications channel remain core ancillary requirements of QKD on the typically classical support layer. The White Rabbit (WR) technology, developed at CERN, provides a convenient means to achieve sub-nanosecond timing synchronisation over optical fibre. In order to enhance its suitability as an ancillary system to QKD, we demonstrate a significant extension of the range of WR over a single uninterrupted stretch of fibre to 250 km, and report on our success in transferring the timing accuracy of WR to coordinating a simultaneous 'start time' between Alice and Bob.