In this project, we hope to apply ideas from integrated photonics, mainly strong index confinement in nanoscale waveguides, to high frequency (~ 10 GHz) acoustic waves, to significantly enhance the acousto-optic interaction strength [Srinivasan et al., J. Phys. D 2019] and minimize the acoustic dissipation at cryogenic temperatures (4K). This work will be a key first step towards the design of universal quantum transducers (Schuetz et al. Phys. Rev. X 2015) based on surface acoustic waves.

P‌rimary Supervisor: Dr Krishna Coimbatore Balram

Homodyne detectors are used across a range of classical and quantum technologies, including optical neural networks, quantum random number generation, quantum metrology, and quantum computing. A key metric for how well these detectors operate is the bandwidth, which limits a range of functionality depending on the application (e.g. key rate in q. comms, optical bandwidth in sensing, and the clock-rate in computation). The bandwidth of the detector is usually characterised by the speed at which a signal is attenuated by a half (the ‘3dB bandwidth’). Bristol currently holds the world record for the fastest homodyne detector [Nature Photonics (2020): 1-5] which has a 3dB bandwidth of around 2 GHz. Interestingly, these detectors also have non-zero electronic clearance (can see a signal) well beyond this, out to 9 GHz. This project is to investigate what can be gained by taking advantage of this middle ground between the 3-dB bandwidth and full-clearance bandwidth for applications in optical neural networks.

P‌rimary Supervisor: Dr Euan Allen

Top-down semiconductor manufacturing techniques are not broadly applicable to bottom down systems and, for 30 years, researchers have failed to build deterministic systems at the atomic level. Without scalability, atom-like systems can not bring their enormous potential (deterministic photon sources and integrated quantum memories) to quantum photonic circuits. Our approach is to accept a yield in useful atom-based qubits and harness foundry processes with built-in redundancy to achieve a deterministic integrated atom-like component.

In this, we move from the scientific exploration of single atom-like defects to the technological need to rapidly evaluate O(1000) atom-like emitters per chip. This is typically time consuming owing to the low signal of single quantum emitters. Good photoluminescence spectra from a single quantum emitter can take many minutes. In this project, we want to explore using compressed sensing to recover the frequency and linewidth of the zero-phonon emission and target a sub-second-per-emitter characterisation. This will give us the ability to network indistinguishable atom-like emitters at scale.

P‌rimary Supervisor: Dr Joe Smith

Hollow core fibers are specialty fibers that exhibit desirable features with a potential to significantly enhance future quantum communications and networking. This comes at a cost of increased optical power losses in the transmission. In this project the student is asked to develop a theoretical model based on VPIphotonics simulation tool to study the trade-off between the absence of non-linearities versus the increased losses and the
effect it might have on the co-existence of classical and quantum channels. Fiber thermal stability and reduced latency features may also be considered for system-level improvements in future quantum networks.

P‌rimary Supervisor: Dr George Kanellos 

Real world quantum communication needs to go beyond two-party protocols. This project will place the student at the forefront of entanglement-based quantum networking. The student will be part of a massive ongoing experiment with 38 international collaborators all coming together to lay the foundations for the quantum internet. In a large-scale experiment taking place in Bristol in early 2021, we will, for the first time ever, create a dynamic software defined quantum network with long distance links, 19 users and advanced control plane architecture. The project places the student in the middle of a truly large scale experiment with several individual aspects. The student is free to choose one or several of these aspects to work on.

P‌rimary Supervisor: Dr Siddarth Joshi

This project contributes to the development of a practical trapped ion quantum computer, focusing on the controlled movement of multiple individual ions around a microfabricated ion trap, in order to facilitate the entangling gates and measurement operations required in quantum error correction.

P‌rimary Supervisor: Professor Winfried Hensinger

Both classical and quantum cryptography rely on random numbers as a basic resource. Historically, either classical processes or classical noise have been used as seeds for pseudo random number generators to generate random bit strings. Being deterministic algorithms, however, the ‘random’ numbers created will have certain correlations within it. This is where quantum random number generators (QRNGs) come in: Using provably random quantum processes, one can also create random numbers, one simple example being a photon at a 50:50 beamsplitter. However, physical implementations of such QRNGs also introduce imperfections due to technological limits. In quantum key distribution this is constantly being accessed and both the implementations and their security proofs are regularly adjusted. QRNGs may be vulnerable to similar or unique types of attack, but a strong overview of such attacks has not been created, yet. In this project B, the student would study QRNG types and attacks described in literature to create an overview of vulnerabilities that should be considered when designing a QRNG.

P‌rimary Supervisor: Professor John Rarity

This project investigates the role of reference-frames in quantum communication protocols based on quantum steering. This open question is of particular interest to the broader community because of its foundational significance and its practical implications for the secure distribution of entanglement and trusted correlations in quantum communication networks.  Within the hierarchy of inseparable quantum correlations, quantum steering is intermediate to entanglement and Bell nonlocality. Whilst Bell protocols are desirable for applications and tests, they require complex and expensive experimental efforts. Steering tests allow us to overcome these limitations due to their inherent robustness to imperfections in state preparations and measurements. Recently, we experimentally demonstrated different promising classes of steering criteria that allow us to bring steering tests closer to practical applications. We extended this work by further testing the robustness of our protocols to misalignment in the measurement reference frame – a resource intensive challenge for emerging fields such quantum fibre networks and ground-to-space communication. This project aims to build up the former work by generalising our previous results and helping to identify most suitable tests for quantum steering. The expected theoretical results can be verified against available experimental data.

Primary Supervisor: Dr Sabine Wollmann

In Four Wave Mixing based sources filtering of the pump signal is of critical importance to prevent pump light reaching sensitive single photon detectors. This requires the use of very high (>110dB) pump rejection filters. This is very challenging to achieve in a single chip design and this project will attempt to understand the fundamental limits of what is achievable in the SOI platform.

Primary Supervisor: Professor Martin Cryan

Single photons are a key enabler of quantum technologies: a push-button box that delivers entangled photons “on demand” into an optical fibre would be valuable for quantum security and many proposals for quantum computing. This project will design and develop such a box, based on cutting edge physics and established semiconductor technology.

Primary Supervisors: Dr Andrew Young, Professor Ruth Oulton

Operator scrambling had been thought of as a characteristic feature of quantum dynamics. For generic quantum dynamics one expects this time evolution to be `chaotic' - in the sense that an initially simple operator will eventually become scrambled amongst a large number of degrees of freedom of the quantum system. This behaviour also occurs in typical quantum circuits, and was previously thought to be intimately connected to the fact that typical quantum dynamics is not classically simulable. Thus our recent discovery of a family of circuits that provably scramble but are also efficiently classically simulable is highly surprising. We call these circuits `super-Clifford circuits', since the Heisenberg time evolution of these operators corresponds to a Clifford evolution in operator space. These circuits provide a new technique for studying scrambling in systems with a large number of qubits, and are an explicit counter example to the intuition that classical simulability implies the absence of scrambling. Our paper produced a single family; but is clear that we have found but one example of a general class. However we do not have a good understanding of how large this class is, or what characterises these simulable scrambling models. The project will find new examples of circuits for which operator scrambling is classically simulable, and provide a general characterisation of such circuits. A further goal is to investigate the rate of scrambling in these circuits. Preliminary [unpublished] results suggest our family of models include “fast scramblers” – circuits that scramble operators as quickly as possible. This is extremely tantalising as models with this characteristic seem to be central to connections made recently between quantum information and quantum gravity. This project will therefore probe this issue, initially using computer simulations to compute the scrambling time. If time allows, next steps could include seeing what new insights this might offer for quantum gravity or other fast scramblers.

Primary Supervisors: Professor Noah Linden, Dr Mike Blake

Optimal control techniques enable us to accurately control quantum system, but given the limitations of simulating quantum systems with classical computers, it is getting increasingly important to implement the design of optimal control sequences directly in the experiment. Many figures of merit, such as gate fidelities, can be experimentally assessed only in terms of a large number or observables. An algorithm that requires assessing the full set of observables in each iteration implies a large experimental effort. Goal of this project is to extend a control algorithm based on statistical machine learning to work also with a reduced number of experimental observations.

Primary Supervisor: Professor Florian Mintert

This project will involve working with single-photon sources based on engineered optical cavities to maximise the visibility of inter-source HOM interference. The main aim for this project is to observe high-visibility HOM interference, beyond the 92\% that micro-racetrack (or otherwise) resonators have been bound to in the past. Once this has been achieved, the second goal will be to work through the process for encoding bell-states using two heralded single-photons, and taking measurements that could contribute to the final results.

Primary Supervisor: Dr Jorge Barreto

Understanding and quantifying the effects of noise and other open systems effects is crucial for developing practical quantum technology. Open systems models are usually impossible to solve analytically without making many approximations which limit their range of validity. And they are challenging computationally for all but small system sizes. A number of tool kits exist to tackle various open systems quantum simulations, and this project aims to evaluate one of the most recent, arXiv:2011.14046, Chen & Lidar. The took kit is written in the Julia programming language, suitable for high performance computing, and contains several master equation models, including the Redfield, the polaron transformed Redfield, and the Lindblad. A key feature is the ability to handle time-dependent Hamiltonians. The aim is to benchmark the performance of the different models using a simple test system, and assess its suitability for different experimental settings. This is a good preparation for research into open systems effects in a wide variety of settings in quantum technology.

Primary Supervisor: Dr Elizabeth Kendon