A practical quantum simulator: simulating molecular vibrations with photons

Computer simulations of physical models have become a vital tool in science and engineering. For example, the aerodynamics and chassis integrity for a new car design will be fully simulated on a computerised model, long before production begins, while biologists will use a simplified computer model to simulate the dynamics involved in protein folding. In both of these cases, the physics underlying the model to be simulated is that of the familiar, classical world, as is the information that is processed. In contrast, chemists working with systems at the microscopic scale (quantum chemists) must incorporate quantum physics into their physical models. But these models come up against the intractability of simulating even modestly sized quantum systems on classical computers.

The number of possible configurations of any system grows exponentially with its degrees of freedom, just like the number of heads/tails configurations of a row of coins doubles with each additional coin. Since a quantum system can exist simultaneously across all of its configurations, its evolution is too large to be simulated with a classical computer. Therefore, quantum mechanical models for classical computers are necessarily limited while more compete models are fundamentally intractable to classical simulation. Yet increasingly, scientists need to understand the role of quantum physics, for example in biological molecules.

The famous physicist and Nobel Laureate, Richard Feynman, identified this problem in a seminal lecture in 1982. He also proposed a solution. Feynman suggested using one controllable quantum system to simulate the model for the quantum system one wishes to study. The ultimate realisation of this ingenious concept is a digital quantum simulator that theoretically can be programmed to simulate any quantum system. Building this device is the focus of an increasingly intensive international effort, or competition. This effort is likely to be long term since isolating, digitising, and coherently controlling large quantum systems has proved to be highly challenging, due to their inclination to couple to the environment, decohere, and behave classically. After all, the world we see around us is classical, not quantum. Therefore, the road to a quantum simulator that surpasses the capabilities of classical computers seems, long and difficult, and is an ultimate goal to scientists working in quantum information science.

This fellowship proposes a smart route to large-scale quantum simulations that is intrinsically scalable, and can be implemented with manufacturable technologies. The project aims to simulate quantum physical models at a scale that surpasses the capabilities of conventional computers. This is possible because a mapping has been identified between an established model for the quantum vibrational behaviour of molecules, which cannot be simulated with a conventional computer, and the description of photons in manufacturable optical chips. By injecting ensembles of single photons into a versatile optical chip, the evolution of a large molecule can be tracked.

The direction of the research is to then make improvements to the molecular mathematical model with a series of perturbations, which, in loose terms, are matched by perturbations to the optical circuits in the form of weak interactions between the photons. The difficulty in getting single photons to strongly interact is the main challenge for optical quantum computers. However, developing successive generations of devices that build up layers of weak interactions allows interesting and complex simulations to be performed on an increasingly tailored and accurate molecular model. As these devices progress, they will develop additional computational capabilities, such as the calculation of factors involved in chemical transitions and characteristic properties of biotic molecules.

Beyond research circles that are already engaged with developing quantum technologies, industrial and technology sectors, the UK economy, and people in the wider society will experience significant impacts from the proposed research.

By investing £270M in developing quantum technologies, the UK government has recognised the possibilities for positive strategic impact on the national technological capability, and the unacceptable deficiency that would result from falling back in the race to realise the key demonstrators. This research will meet this national strategic requirement by targeting intrinsically efficient quantum simulations for a manufacturable technology, where classical computers fail. The international profile of UK technologies will experience a significant boost, as will the prospects for export, and the endorsement of government's investment, which will likely translate to a policy of deeper investment.

The impact from the arrival of physical devices with algorithmic capabilities that are impossible to match by any upgrade or improvement to classical technology will be felt across computer science and the ICT sector, including high performance computing and software developers. Stakeholders in these areas are likely to want early exposure to the new quantum technology. As a key demonstrator, the proposed devices will be available to computer scientists and programmers as a testbed for the development of quantum coding languages.

New applications for industrial quantum chemists will be related to calculation of currently intractable factors involved in predicting electronic transitions. The simulators will engage biologists in the effort to understand the role of quantum mechanics in biology, and to simulate proposed quantum models for biotic molecules and proteins. With lattice vibration models central to condensed matter, physicists in this area, together with chemists and biologists, can explore further simulator applications by working with prototypical devices and further engage in the development of quantum technologies.

Growth should be anticipated in the photonics technologies industrial sector, as quantum photonics technologies translate from the lab to find applications in the wider world. An IP portfolio for photonic quantum technologies is expected from this research, with guidance from the University of Bristol Research Commercialisation Team. I have previous experience of this process with a joint quantum communication patent with Nokia.

For people in the wider society, the profile of quantum information processors will be catapulted into the collective consciousness in a way that can only happen with a tangible physical demonstrator. The new capability will motive the inclusion of quantum information science in the classroom, particularly as part of traditional IT lessons. The scientific cultural landscape of the general public in the UK, particularly among popular science enthusiasts, will buzz with excitement as the research outputs are reported by mainstream media. There will follow a desire to understand something of the new technological capabilities and the underlying quantum physics. Interest in studying physics beyond school level should be boosted.