Measurement-based entanglement of single-dopant As spin qubits

The aim of this research project is to demonstrate an all-electrical and scalable measurement-based or one-way quantum processor, through using a measurement-based approach to generating entanglement between qubits. Single arsenic dopant spin qubits in silicon will be used in the processor. The spin of an electron is a natural basis for a two-level system qubit. The individual energy levels are easily controllable by magnetic fields through the Zeeman effect, and isolation and manipulation of individual qubits is possible through the electron's charge. Electron spin qubits are often implemented in silicon-based devices as they can be more easily integrated with conventional CMOS technology, and are usually implemented as quantum dots defined by gate electrodes, or as dopants incorporated into the silicon. Phosphorus dopants in silicon have reported long coherence times in excess of 0.5 seconds, and are a promising platform for a quantum computer.

Rather than phosphorus, the dopants used will be arsenic. Arsenic dopants have a I = 3/2 nuclear spin manifold, allowing 4-state Zeeman splitting under a magnetic field and d = 4 qudit encoding in nuclear spins. With the much longer nuclear spin coherence times in excess of 30 seconds, the storage of quantum states in the nuclear spins of dopants as a form of quantum memory is made possible through the hyperfine coupling between electron and nuclear spins. The storage of states is key to the realisation of the measurement-based quantum computer which relies on an initial resource state such as a cluster state, a highly entangled cluster of qubits that will require sufficient time to grow. Entangling measurements between qubits will be realised using reflectometry qubit readout techniques.

Measurement-based entanglements also open the door to all-to-all connectivity between qubits, overcoming the nearest neighbour limitation for two-qubit gates based on exchange interactions in the more conventional method of gate-based quantum computation. This will give full flexibility in forming the resource state for a universal quantum computer.

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.