Quantum Networking through Polarisation Entanglement Distribution

Abstract

Quantum Technology is rapidly developing. To scale the technology beyond single monolithic systems, methods of interconnecting many systems are required. It is no longer a question of if interconnects will be used, but is now a question of when. The prevailing consensus is to interconnect quantum systems through the distribution of entanglement. This entanglement can then be used as a resource, and can be distributed through entanglement-based quantum networks (EQNets). Many different methods of constructing EQNets have been presented, however there is yet no consensus on whether any single design of system can fulfil the needs of all quantum technologies. In this work I present a type of fibre based EQNet, based on wavelength multiplexing of polarisation entangled photon pairs.

A current realisable application of these EQNets is quantum key distribution (QKD). As such I use QKD as a performance metric of the quality and quantity of entanglement distribution (ED) in the polarisation-entanglement-based quantum networks (PEQNets) I constructed. I first demonstrate record uptime of a passively polarisation compensated fully-connected PEQNet, to date of publication. This achieved a continuous up-time of 10.8 days with metropolitan area optical fibre infrastructure (OFI) links, with 10 users. This allowed all parties to share entanglement throughout the demonstration with all other parties in the PEQNet, simultaneously in pairwise engagement links. To complete this work a quantum-enabled reconfigurable optical add-drop multiplexer (q-ROADM) was designed to dynamically assign all resources, such that the resourced can be dynamically reassigned. This is the first demonstration of an EQNet using a q-ROADM to distribute the entanglement resource.

Interconnecting EQNets allows for ED between larger sets of users. Towards this goal of interconnectivity, a set of 2 PEQNets is demonstrated, where one is constructed with 16 users and the other with 3 users. The 3 user PEQNet is connected with OFI links, where the longest is a 48.9 km link. Both PEQNets were established, where all links were able to perform QKD. Dynamic, interconnecting, links were then added to the two PEQNets. This demonstrated that such PEQNets can be interconnected to share entanglement beyond a single PEQNet. In these demonstrations the 16 user PEQNet shows long-term stable uptime, repeating the previous results, however the long-distance links showed significantly reduced stability, with a maximum up-time of 5 hours. A method for continuously compensating for the polarisation state is then explored, showing that a feedback system can compensate for polarisation rotations in OFI.
Finally, collaborative work toward optimisation, of both resources and infrastructure, is presented. To optimise the resources, dynamic allocation of the entanglement resource can be used to improve the secret key (SK) generation of QKD in these PEQNets. Detail is then given about how the optimum resource allocation is affected by parameters of the system, such as heralding efficiency of the system and the jitter of the single-photon detectors used. To optimise the use of infrastructure, going beyond just quantum signals in OFI is required. As such analysis of co-existence of both quantum and classical optical signals is presented. Then methods to practically combine these optical signals into the same infrastructure is demonstrated in both OFI type links, and hollow core fibre (HCF) links. This HCF would allow for co-existence of bright classical communications without the broadband scattering of light, as seen in OFI. Work towards large scale heterogeneous quantum networks (QNets) is then detailed, showing that co-existence of different quantum resources, and additional classical resources, can work in a single link within a QNet.

Date of Award	23 Jan 2024
Original language	English
Awarding Institution	University of Bristol
Sponsors	Quantum Communications Hub
Supervisor	Siddarth K Joshi (Supervisor) & John G Rarity (Supervisor)