Integrated Quantum Frequency Combs for Cluster States Generation

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Quantum computing, once a purely theoretical dream, is becoming a reality. The last two years - 2019 and 2020 - have seen the first demonstrations of quantum advantage, that is, quantum computers performing tasks that cannot be solved by any classical supercomputer in a reasonable time. Among the competing technologies, photonics and superconducting circuits are the only two that reached quantum advantage. In both cases, the challenge ahead is scalability: the current quantum machines can only process a limited number of quantum bits, limiting their application to the solution of proof-of-concept problems.This project aims to develop a highly complex quantum state, called cluster state, that underpins a scalable approach to photonic quantum computing. Such a state, to be useful, must live on miniaturised components fabricated by a mature technology that supports scalability with increased complexity and commercial viability. Integrated photonics satisfies both these requirements. It can count on a large selection of tools and devices developed during the last forty years for telecommunications and data processing and employs the latest technologies that connect classical computers.This project will use the latest discoveries and technologies of integrated photonics for realising the generation of the aforementioned cluster states on-chip.One of the more recent and striking successes of integrated photonics is the realisation of frequency combs on-chip (microcombs). Frequency combs, electromagnetic fields composed by many equidistant frequencies (light colours), are a powerful resource for metrology and spectroscopy. Their miniaturisation via integrated photonics transformed them from cumbersome bulk systems to few millimetres squared chips, making integrated frequency combs one of the most promising photonic technologies.Frequency combs feature thousands to millions of modes and land themselves naturally to host the cluster states required for scalable quantum computation. To transform an integrated frequency comb into a cluster state all its frequency modes must be entangled, that is, sharing the same quantum state. Here, I propose a research programme that tackles this very problem: generating an integrated cluster state based on frequency combs.The research developed in this proposal will disclose the potential of integrated quantum frequency combs for quantum computing. This will boost a number of applications identified as strategic by the UK Government and Research Councils such as clock synchronisation and the deployment of quantum technologies.