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Our students undertake research across diverse topics, see below for examples of PhD projects.
Experimental
- Digital Control of Superconducting Fluxonium Qubits | Leon Guerrro
Industry Sponsor: SeeQC
Primary Supervisor: Prof. Paul Warburton
- Light Cones in Quantum Matter | Tom Holden-Dye
Primary Supervisor: Dr. Arijeet Pal
Secondary Supervisor: Dr. Lluis Masanes
Abstract: Quantum theory is perhaps our most successful theory of Nature - it describes the microscopic laws underpinning the dynamics of atoms and subatomic particles with truly remarkable precision. One might think that, armed with these laws, we should have no problem predicting the phenomena and behaviour that emerges from the motion and interaction of these particles. But this is not the case! The mathematical structure of quantum theory, in general, makes it very hard to calculate what happens when lots of quantum particles interact together, even if one uses the world's most powerful supercomputers. A proposed solution to this problem is to use quantum particles themselves to do the calculation - to build a quantum computer, which could simulate the quantum world in far greater detail than is currently possible. But we still need to understand (at least relatively well) how the large, many-particle quantum systems needed to build such a device behave in order to realise this. The goal of this PhD project will be to develop new algorithms - for classical and quantum computers - which can efficiently calculate properties of many-body quantum systems. In particular, I will focus on the problem of characterising how quickly information can spread out in different quantum systems. This may help us benchmark and test quantum computers more robustly, study protocols for correcting errors in quantum computers, and tackle open problems in many-body quantum physics - such as understanding interesting phase transitions between states of many-body quantum systems with drastically different behaviour - in a new way. Visit UKRI website to find out about planned impact.
- Towards a Logical Qubit in Silicon: Scalable 2-qubits Operations in CMOS-compatible Quantum Dots | Constance Lainé
Industry Sponsor: Quantum Motion
Primary Supervisor: Prof. John Morton
External Supervisor: Prof. Fernando Gonzalez-Zalba
- Quantum Error Correction in Materials Simulations | Hasan Sayginel
Industry Sponsor: NPL
Primary Supervisor: Prof. Dan Browne
External Supervisor: Dr. Ivan Rungger
- Intrinsically Protected Superconducting Quantum Circuits | Yi Shi
Primary Supervisor: Prof. Marzena Szymanska
- Data Structures and Other Building Blocks for Quantum Algorithms | Shashvat Shushvat
Industry Sponsor: Horizon Quantum Computing
Primary Supervisor: Prof. Dan Browne
External Supervisor: Joe Fitzsimons
- Long-Range Exchange Coupling between Ge-Hole Spin Quantum Systems | Martyna Sienkiewicz
Primary Supervisor: Dr Stuart Holmes
- Measurement-based Entanglement of Single-dopant As Spin Qubits | Matthew Tam
Primary Supervisor: Dr. Mark Buitelaar
Non-experimental
- Parametric Amplification in Nanobridge SLUG Embedded in Microwave Cavity | Parth Bhandari
Industry Sponsor: National Physics Lab
Primary Supervisor: Dr Edward Romans
External Supervisor: Prof Ling Hao
Abstract: The rapidly evolving field of hardware for quantum computation has led to significant growth in developing low-temperature amplifiers. The interest in quantum amplifiers has increased because of the demand for low-noise amplifiers operating at cryogenic temperatures for other sensitive applications. In the thesis, we will focus on the development of quantum amplifiers with a working range near 26 GHz that are required for the readout chain in future planned experiments to determine the mass of electron neutrinos via cyclotron radiation spectroscopy. We aim to develop both Superconducting Quantum Interference Devices (SQUIDs) and Superconducting Low-inductance Undulatory Galvanometers (SLUG) as amplifiers embedded in a microwave circuit. SQUIDs and SLUGs with conventional Josephson junctions have previously been coupled with RF-microwave circuits, and in this study we will extend this work by developing SLUGs with nano-bridge junctions. This will lead to a simpler fabrication process for quantum amplifiers and a higher operating frequency as compared to current state-of-the-art quantum amplifiers.
- Phases of quantum advantage: A proposal for the study of critical non-unitary dynamics to improve algorithm design | Charlie Solomons-Tuke
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- Quantum Approximation Optimisation Algorithm for Error-Correcting Codes | Sheila Perez Garcia
Industry Sponsor: Phasecraft - Ashley Montanaro
Primary Supervisor: Prof Toby Cubitt
External Supervisor: Dr Lluis Masanes
Abstract: Quantum computers have the potential to solve problems that classical computers cannot address feasibly. However, the current limitations of quantum hardware make building a fully operational quantum computer challenging. Therefore, focusing on algorithms suitable for the current generation of Noisy Intermediate-Scale Quantum (NISQ) computers is essential to finding practical quantum computing applications. The Quantum Approximate Optimization Algorithm (QAOA) is a hybrid quantum algorithm that combines quantum computing with classical optimisation. It is designed to solve combinatorial optimisation problems, which involve finding the optimal arrangement of variables subject to specific constraints. Classical linear error-correcting codes are used to transmit encoded signals through noisy communication channels, where they detect and correct errors in the message. These codes can be seen as constraints satisfied by the codewords. Decoding a bit string with errors thus becomes a maximum-likelihood decoding problem, aiming to find the codeword closest to the received bit string. This PhD project is dedicated to a comprehensive analysis of QAOA's performance in decoding linear error-correcting codes. The aim is to pinpoint areas where QAOA may surpass classical decoding algorithms and address some of their inherent limitations.
- Noise and fault resilience of dissipative algorithms | James Purcell
Primary Supervisor: Prof Toby Cubitt
External Supervisor: Dr Lluis Masanes
Abstract: The engineering of exotic quantum materials will be of major importance in the coming years. To properly exploit this new paradigm will require a deeper understanding of the physics of many-body quantum systems. This is challenging - quantum systems are highly complex and contain phenomena not found in classical systems. Perhaps, as a quantum system itself, quantum computers are able to render tractable some questions about physical systems that cannot be tackled classically. However, it is important to understand when in fact this cannot happen. A natural lens for such an understanding is that of the computer science notions of computability and complexity. Many examples of systems that have been constructed to have behaviour that is provably hard (or impossible) to predict are highly artificial and the relevance to systems found in nature, or to systems which we can engineer, may be somewhat questionable. This project will explore the extension of hardness results to 1) more realistic systems, and 2) simpler systems.
- Quantum gravity simulation on a quantum computer | Anastasia Moroz
Primary Supervisor: Dr Lluis Masanes
External Supervisor: Prof Toby Cubitt
Abstract: A unified theory encompassing general relativity and quantum mechanics has remained elusive since research began in the 1930s. Current approaches addressing this challenge have progressed slowly and have made limited predictions for this theory. However, the recent application of tensor networks to the Anti-de Sitter/Conformal field theory (AdS/CFT) duality offers a fresh and promising route to gain insights into quantum gravity. This duality conjectures that quantum gravity theories defined in asymptotically anti-de Sitter spaces of d + 1 dimensions are dual to conformal field theories defined on their d-dimensional boundaries. By leveraging tensor networks, the entanglement structure and the emergence of spacetime geometry can be effectively captured. The mathematical tractability of discrete tensor networks also circumvents the complexities inherent in other approaches to quantum gravity. A discrete spacetime realisation of the AdS/CFT correspondence would useful for the development of quantum error correcting codes and for providing a numerical framework to study strong interactions. The objective of this PhD research is to investigate whether conformal dual-unitary circuits constitute a discrete spacetime realisation of the AdS/CFT correspondence and serve as a viable toy model of quantum gravity. This investigation will involve characterising the underlying physics of this model and developing methods for its efficient simulation on a quantum computer.
- Stability of Dynamic Quantum Phases in Higher Dimensions | Marcell Kovács
Primary Supervisor: Prof Arijeet Pal
External Supervisor: Prof Sougato Bose
Abstract: Understanding the dynamics of non-equillibrium many-body systems is a contemporary challenge in quantum theory. This project investigates non-ergodic phases arising in disordered systems, in which the relaxation to an equillibrium state is inhibited as the system evolves. In particular, we study whether such behaviour is robust against defects and perturbations and use the toolkit of quantum information theory (e.g. entanglement measures and spectral probes of quantum chaos) to characterise time evolution. The quantum circuit model is used as an analytically tractable limit of interacting quantum dynamics where thermalisation can be rigorously investigated. We plan to further study the stability of many-body phases beyond this setting, such as in the context of quantum simulation platforms to form a link with contemporary experimental efforts.
- Measurement-based quantum computation and entanglement generation using carbon nanotubes and As dopants in silicon devices | Torr Fischman
Primary Supervisor: Dr Mark Buitelaar
External Supervisor: Prof Paul Warburton
Abstract: The overall aim of this PhD project is to demonstrate measurement-based quantum computation (MBQC) and entanglement using two different types of devices: carbon nanotubes (CNTs) and further into the Phd, silicon (Si) with arsenic (As) dopants (Si:As) devices. The former will be used during the first year of the PhD to develop the radio frequency (rf) reflectometry readout and microwave control qubit techniques on these highly developed devices. The single As dopants in Si will use the same control and measurement techniques to finally demonstrate measurement-based entanglement generation and quantum computation with all-electrical control. It is expected that this approach will provide a means for scalable distribute quantum computation. The idea behind the As dopants is that they can provide a quantum memory to store quantum information via their (I=3/2) nuclear spin states. At the same time, the electron spins will be used for the projective measurements that generate the remote entanglement of the As dopants. Hence, quantum information can be swapped between the electron and nuclear spins. Ultrafast rf reflectometry and microwave techniques will be used for qubit readout and control, respectively.
- Scalable two-dimensional silicon quantum processor | Jeremy Morgan
Industry Sponsor: Quantum Motion
Primary Supervisor: Prof John Morton
Supervisor: Fernando Gonzalez-Zalba
Abstract: Quantum computing has the potential to solve a wide range of problems with applications in drug discovery, materials simulations and machine learning. Large quantum computers could perform some calculations much faster than conventional computers and have the potential to significantly outperform normal computers in a select range of problems. There are many platforms that can be used to build a quantum computer, and there is currently no outright solution. A natural candidate is the use of electron spins, known as spin qubits. Silicon spin qubits are promising due to their high qubit density and compatibility with standard industrial processes. However, spin qubit systems are relatively immature in comparison to other hardware platforms and require scaling. This PhD project will investigate a scalable two-dimensional design for spin qubits in silicon using industrially fabricated devices ensuring that the devices are compatible with mass scale production techniques. This will require careful consideration of individual control and readout of qubits as the devices scale in two dimensions. This presents a challenge due to the sensitivity of qubits where interactions with the local environment can cause a loss of information and therefore the impact of multiple qubits interacting with each other may present a significant challenge. This PhD will leverage the expertise and knowledge in Quantum Motion on the sensing and control of linear arrays of silicon spin qubits with the aim of demonstrating a scalable design for a 2xN spin qubit array using industrially fabricated devices. The focus of this PhD project is on the development of scalable dense two-dimensional architectures in silicon quantum dot devices fabricated in an industrial foundry. The central aim is to develop techniques and protocols to initialise and operate complex quantum dot devices culminating in the realisation of a 2xN quantum processor. Initially, the project will focus on demonstrating single-qubit gates, implemented with electron spin resonance (ESR) via on-chip microwave transmission lines. The demonstration of single-qubit operations with ESR will allow the device initialisation and readout methods to be validated. Subsequently, two qubit gates can be implemented in quantum dot devices through the exchange interaction between neighbouring electrons. Two-qubit operations can be performed in different ways, either by pulsing the gates controlling the barrier and plungers of the electrons, performing the √SWAP operation or through the application of resonant microwave pulses to an ESR line in an exchange-coupled two-qubit system, implementing controlled rotation (CROT) gates. This is the conditional rotation of one qubit based upon the state of the other. In order to demonstrate high fidelity qubit operations, accurate readout methods are also required. The measurement of spin qubits can be performed using charge sensing such as with single-electron-transistors (SET) combined with spin-to-charge conversion techniques such as Elzerman readout and Pauli-Spin-Blockade. The large footprint of SET’s is not scalable and therefore charge sensing using single-electron-boxes (SEB’s) and gate-based dispersive sensing can be used to reduce the on-chip readout footprint required. By demonstrating single- and two-qubit operations in combination with scalable readout techniques and initialisation protocols, the essential components for a scalable quantum processor will be realised in this project.
