Integrated Optics as a Platform for Continuous Variable Measurement Based Quantum Computing

Recently there have been demonstrations of squeezing using Lithium Niobate and Silicon-based integrated chip architectures[5]. These results have been claimed to be useful for quantum computing, however the squeezing levels achieved so far are low. Also recently, extremely large 2D continuous variable cluster states have been generated using bulk optics[1,2]. A key element of these demonstrations were large temporal delays to enable the use of time-bin encoding. Increasing the number of input modes in the cluster requires an increase in the delay, constraining the scalability of an input register. This PhD aims to explore the generation of CV cluster states using integrated optical technology with the view to designing a scalable integrated platform.
A key experimental challenge to be addressed is the reconciliation of the squeezing levels required for cluster state verification (~3-5dB) and full fault tolerance (~15-17dB [7]) with current integrated results of ~1-2dB. A first stage of the PhD would therefore be to use commercially available lithium niobite modules coupled into a chip with integrated Homodyne detectors to observe the minimum 3dB of squeezing required. The PhD is sufficiently flexible to pivot and use demonstrated silicon and silicon-nitride based squeezing if they meet requirements.
Once this has been achieved, a milestone of the PhD will be to understand the theory of, design, construct and verify a 1D integrated massive cluster state generation platform based upon existing bulk schemes[3]. A stretch goal of this project would then be to implement gaussian operations on the cluster state to demonstrate a computing proof of concept following recently established techniques[4]. A second targeted project will be to extend the cluster state to 2D. However, due to the large delays required (and associated losses) in the double time-bin encoding method an alternative platform using frequency and/or spatial modes will need to be explored[6]. This sub theme of the PhD will constitute multiple possibilities for projects, including performing state teleportation between two spectrally separate frequency modes and using the generated experiments for multi-partite entanglement demonstrations. Finally, an alternate direction for the PhD would be to look at optical generation of required non gaussian states for fault tolerance in CV computation.
In addition to the experimental projects, the PhD will also consist of a significant theory component relating to Fault Tolerance for Universal CV-MBQC. To take advantage of error correction techniques, a bosonic code is needed to embed a qubit within the continuous system. Furthermore, to enable universality a Non-Gaussian operation is needed. Both of these challenges can be resolved by the use of 'GKP' qubits which are injected into the cluster states[8]. A collaborative effort with Nicolas Menicucci is proposed to directly address some of these challenges by studying mixed Gaussian / GKP encoding schemes. The proposed collaboration will involve a secondment at RMIT within Nick's group for 3-6 months to develop this project.

1. Deterministic generation of a two-dimensional cluster state https://science.sciencemag.org/content/366/6463/369
2. Generation of time-domain-multiplexed two-dimensional cluster state https://science.sciencemag.org/content/366/6463/373
3. Generation of one-million- mode continuous-variable cluster state by unlimited time-domain multiplexing https://aip.scitation.org/doi/10.1063/1.4962732[4].
4. One-hundred step measurement-based quantum computation multiplexed in the time domain with 25 MHz clock frequency https://arxiv.org/pdf/2006.11537.pdf
5. Single-mode quadrature squeezing using dual-pump four-wave mixing in a nanophotonic device https://arxiv.org/abs/2001.09474
6. https://arxiv.org/pdf/1912.11215
7. https://journals.aps.org/pra/abstract/10.1103/PhysRevA.100.010301
8. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.200502

Our ambitions for the impact of the Quantum Engineering CDT are simple and clear: our PhD graduates will be the key talent that creates a new, thriving, globally-competitive quantum industry within the UK. In Bristol we will provide an entire ecosystem for innovation in quantum technologies (QT). Our strong and diverse research base includes strengths going from quantum foundations to algorithms, experimental quantum science to quantum hardware. What makes Bristol unique is our strong innovation and entrepreneurship focus that is deeply embedded in the entire culture of the CDT and beyond. This is reflected in our recent successful venture QTEC, the Quantum Technologies Enterprise Centre, and our Quantum Technologies Innovation Centre (QTIC), which are already enabling industry and entrepreneurs to set up their own QT activities in Bristol. This all occurs alongside internationally recognised incubators/accelerators SetSquared, EngineShed, and UnitDX.

At the centre of this ecosystem lies the CDT. We will not just be supplying existing industry with deeply trained talent, but they will become the CEOs and CTOs of new QT companies. We are already well along this path: 7 Bristol PhD students are currently involved in QT start-ups and 3 alumni have founded their own companies. We expect this number to rise significantly when the first CDT cohort graduates next year (2 students have already secured start-up positions). Equally, it is likely that our graduates will be the first quantum engineers to make new innovations in existing classical technology companies - this is an important aspect, as e.g. the existing photonics, aerospace and telecommunications industries will also need QT experts.

The portfolio of talent with which each CDT graduate will be equipped makes them uniquely suited to many roles in this future QT space. They will have a deep knowledge of their subject, having produced world-leading research, but will also understand how to turn basic science into a product. They will have worked with individuals in their cohort with very different skills background, making them invaluable to companies in the future who need these interdisciplinary team skills to bring about quantum innovations in their own companies. Such skills in teamworking, project management, and self-lead innovation are evidenced by the hugely successful Quantum Innovation Lab (QIL). The idea and development of QIL is entirely student-driven: it brings together diverse industrial partners such as Deutsche Bank, Hitachi, and MSquared Lasers, Airbus, BT, and Leonardo - the competition to take part in QIL shows the hunger by national industry for QT in general, and our students' skills and abilities specifically. With this in mind, our Programme has been co-developed with local, UK, and international companies which are presently investing in QT, such as Airbus, BT, Google, Heilbronn, Hitachi, HPE, IDQuantique, Keysight, Microsoft, Oxford Instruments, and Rigetti. The technologies we target should lead to products in the short and medium term, not just the longer term. The first UK-wide fibre-based quantum communication network will likely involve an academic-industrial partnership with our CDT graduates leading the way. Quantum sensing devices are likely to be the product of individual innovators within the CDT and supported by QTIC in the form of spin-outs. Our graduates will be well-positioned to contribute to the advancement of quantum simulation and computing hardware, as developed by e.g. our partners Google, Microsoft and Rigetti. New to the CDT will be enhanced training in quantum software: this is an area where the UK has a strong chance to play a key role. Our CDT graduates will be able to contribute to all aspects of the software stack required for first-generation quantum computers and simulators, the potential impact of which is shown by the current flurry of global activity in this area.