Scalable two-dimensional silicon quantum processor

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-electrontransistors (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.

The first and most important impact of our Centre will be through the cross-disciplinary technical training it provides for its students. Through this training, they will have not only skills to control and exploit quantum physics in new ways, but also the background in device engineering and information science to bring these ideas to implementation and to seek out new applications. Our commercial and governmental partners tell us how important these skills are in the growing number of people they are hiring in the field of quantum technologies. In the longer term we expect our graduates to be prominent in the development of new technologies and their application to communication, information processing, and measurement science in leading university and government laboratories as well as in commercial research and development. In the shorter term we expect them to be carrying out doctoral research of the highest international quality.

Second, impact will also flow from the students' approach to enterprise and technology transfer. From the outset they will be encouraged to think about the value of intellectual property, the opportunity it provides, and the fundraising needed to support research and development. As students with this mindset come to play a prominent part in university and commercial laboratories, their common background will help to break down the traditional barriers between these sectors and deliver the promise of quantum technologies for the benefit of the UK and world economies. Concrete actions to accelerate this impact will include entrepreneurship training and an annual CDT industry day.

Third, through the participation it nucleates in the training programme and in students' research, the Centre will bring together a community of partners from industry and government laboratories. In the short term this will facilitate new collaborations and networks involving the partners and the students; in the long term it will help to ensure that the supply of highly skilled people from the CDT reaches the parts of industry that need them most.

Finally, the CDT will have a strong impact on the quantum technologies training landscape in the UK. The Centre will organise training events and workshops open to all doctoral researchers to attend. We will also collaborate with CDTs in the quantum technologies and related research areas to coordinate our efforts and maximise our joint impact. Working in consort, these CDTs will form a vibrant national training network benefitting the entire UK doctoral research community.