On Chip Generation and Characterization of Non-Gaussian States of light

Non-gaussian states of light have tremendous applications across quantum technologies; especially as a way of introducing the required non-gaussian elements for continuous variable (CV) methods to gain a true quantum advantage. The aim of this project is to work on their generation, detection and manipulation using integrated photonics.

Schrodinger Cat states are an example of such a non-gaussian state (see [1] for more information on cat states) and can be generated in a plethora of ways. For this project the method of generation is via photon subtraction. Generation of cat states is achieved by producing a squeezed state from which a single photon is removed and detected. This is done using a weak reflectivity beam splitter. By heralding on these events, one knows that what remains of state has had a single photon removed from it and matches the desired state. This process is much simpler than methods proposed for generating other non-gaussian states such as GKP states [2] making these states an attractive option for practical implementations.

Whilst the generation of these states has already been achieved via photon subtraction (for example [3]) it has not been done using integrated photonics. Giacomo has already produced a chip to do this, which is capable of generation, subtraction and detection all on the same chip. If this is successful it would mark the first time a photon subtracted state has been generated using integrated photonics. The huge advantage in terms of stability, footprint and scalability of integrated photonics makes the choice of this platform clear. This project builds on work I have done over project B, where I have been looking at detection schemes for exactly this experiment. The first part of the PhD would be building on top of this work in order to figure out the optimum way to operate this chip as well as taking over from the work Beth has done in her project B to finish the characterisation and setup of the chip.

Overall, it is the setup and operation of this chip that would form the backbone of the PhD. What can this chip do? As already mentioned, this chip generates and detects these photon subtracted states and can be operated in a continuous wave (CW) or pulsed mode. Each of these has their own operational considerations which must be addressed alongside things including mode shaping and timing synchronisation for events. Even if/when cat state are generated from this chip in both the CW and pulsed regime there are more results that can be generated. As this chip has two copies of the structures needed to generate these states one can pump both copies to generate multiple states. Due to the presence of beam splitters and interferometers linking these two structure it is possible to interfere these two states and entangle them opening the door to a wealth of possible experiments. This includes the cat breeding protocol where two of these cat states are joined together to form a larger one [4,5]. By doing this one large state can be generated by combining smaller states which are easier to produce.

It is hoped that once this chip has been used to successfully demonstrate the generation of these states that I would design a new chip to build upon these results- looking at generating new states or applying these cat states in fun ways. Another possible route is to look to build a cluster state [6]. Cluster states are the starting blocks of continuous variable quantum computing using the measurement-based approach and have significant advantages when constructed from non-gaussian states and have yet been created on chip.

Whilst these represent possible future directions it is impossible to say with certainty what may follow on from work on the existing chip but I hope this motivates enough the large variety of possible directions that this work could take.

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.