Beyond binary quantum communication with integrated photonics

Abstract

Quantum technologies have the potential to revolutionise the way we process and communicate information. Such technologies utilise the unique properties of quantum mechanical systems such as entanglement and superposition. While there are multiple potential platforms for quantum technologies, the use of photons, quanta of light, shows promise due to their long coherence time and ease of transmission. In particular, integrated photonic chips are able to generate and manipulate quantum states with high stability and component density, leveraging decades of technological advancements from chip-based electronics.
Current quantum technologies are limited by the small-scale of quantum systems due to their sensitive nature. In this thesis, we present different ways to increase the scale of quantum systems: using higher dimensional encoding, not just binary systems; networking integrated photonic chips; and utilising a novel communication protocol.
We present characterisation techniques that are the backbone of integrated quantum photonics and enable the networking of photonic chips. We introduce a protocol that enables stable communication of high-dimensional, path-encoded states between photonic chips. Using this protocol, we demonstrate the chip-to-chip distribution of four-dimensional states with 97±3% fidelity, improved from 15±14% without the stabilisation protocol implemented. We also propose a new stabilisation protocol that has the potential to improve reliability and bandwidth of chip-to-chip quantum state distribution. We use the demonstrated stabilisation protocol to enable the distribution of high-dimensional entangled quantum states between integrated photonic chips. We discuss how to perform measurement-efficient quantum state tomography, and use this to verify the distribution of high-dimensional quantum states. Without the stabilisation protocol, we measure a fidelity of 8.1% for the distribution of four-dimensional quantum states. We present an improved fidelity of 86% when we implement our stabilisation protocol. We also present progress towards implementing the Floodlight quantum key distribution protocol using integrated photonics, a protocol which has the capability of state-of-the-art communication rates.

Date of Award	4 Feb 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jorge Barreto (Supervisor) & John G Rarity (Supervisor)