Advancing Quantum Chemistry on NISQ Devices

Quantum computers promise to yield solutions to complex problems that have previously been unattainable by classical means. To date, this has been little more than hype. Despite a growing suite of quantum algorithms having been in development since 1992, most require fault-tolerant machines with long coherence times. Such devices are still many decades away, thus relegating the algorithms reliant upon them to theoretical curiosities. Existing and near-term quantum computers are noisy and constrained by short coherence times. So, the question is this: what can quantum computers feasibly do today, or at least in the next few years? The answer lies in quantum chemistry.

The classical computational cost of performing electronic structure calculations grows exponentially with system size, due to an inherent inability of classical hardware to represent quantum states efficiently. In 1981, Feynman remarked: \Nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical". Unlike Feynman, we now have access to quantum computers, yet they have revealed no truths of Nature that were not already known to classical computation. It is vital that practical methods of quantum computing are at the forefront.

Furthermore, such techniques should be scalable, so that as qubit counts increase, so too does the scope for applicability of our algorithms. This PhD project shall provide aid to that end. Simulation methods that facilitate tailoring computational workload to the available quantum resource are in development, though there are a handful of critical issues that need addressing before we may obtain chemically and biologically significant outcomes. This project will drive quantum simulations to unprecedented scales, with the goal of benchmarking these methods against low-hanging fruit such as simulating the proton transfer mechanism in DNA.

Beyond this, we are confronted with the more general problem of simulating chemical reactions within enzymes. This will be achieved via some embedding process, in which we perform quantum simulations on the parts of a molecule that are tractable with respect to existing and near-term quantum computing hardware, and study the rest of the system using the best classical computing approaches to simulating quantum mechanics, such as density functional theory.

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.