Heralded single-photon source with on-chip heralding detector

Quantum interference between single-photons is a prerequisite of an optical quantum computing. This requirement means that we requires bright, deterministic single-photon sources that can consistently produce single-photons which are simultaneously pure and indistinguishable into a single mode. [1,2] Photonic integrated circuit (PIC) technology allows us to engineer nonlinear single-photon pair sources to satisfy all these requirements simultaneously except that they are still inherently probabilistic. Nevertheless, the heralded nature of such sources allows us to multiplex an array of them to achieve a more deterministic logical source [1,3]. Multiplexing, however, requires feedforwarding via electronics which is slow compared to the speed of photons, requiring us to use a long optical delay line, so that the heralding signal can be transduced to a switch controlling signal for the heralded photon in time. The long delay line is undesirable as the longer it is, the more photons are loss in it, and hence less brightness. The first step towards reducing the length of the feedforwarding time delay, and thus the delay line, is to place the detector for the heralding photon as close as possible to the source, or, for PIC-based photon source, on the same chip. This is absolutely unavoidable especially if the end goal is a fully integrated on-chip multiplexing. Furthermore, integrating the heralding detector on-chip would also boost the system heralding event probability as it would eliminate potential sources of the transmission-related loss between the source and the detector, e.g., chip-to-fibre coupler loss, fibre loss, etc.
Single-photons in general may be detected by many means but the only mean that can simultaneously achieve high efficiency, low dark count rate, low reset time, and low jitter is via superconducting nanowire single-photon detectors (SNSPD) with the only expense of having to operate it at cryogenic temperature [3,4]. In our case, the heralding of single-photon source for multiplexing, however, requires all the aforementioned performances [1], making it our only choice. In year 1 of this PhD, I will attempt to integrate SNSPDs directly on top of waveguides, allowing it to be close to the source as motivated above, with the aim of only low-to-moderate performances. The promise of the boost in the system heralding event probability also requires that the filter for the heralding photon incur minimal loss. In addition, to prevent false heralding which will impact the heralding efficiency (the probability of delivering a heralded single-photon given a heralding event), it also needs to maintain the high extinction ratio necessary to reject the noises from the pump and the unwanted non-linear processes [3,5] that may be present in the chip. The development of a range of filtering strategies and designs-based on existing solutions in literature and developed jointly with the SNSPDs fabricated in-house-will be the focus of year 2. If everything goes according to plan, in year 3, the final PIC for a single-photon source with on-chip heralding detector may be designed, fabricated and post-processed, using the components from the previous years, and hopefully, the on-chip heralding will be demonstrated.

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.