Rydberg polaritons in Cu2O microcavities

Exploiting the laws of quantum mechanics for the benefit of society in the so-called "second quantum revolution" is one of the greatest challenges of 21st-century physics. With such capability, we would be able to make quantum technologies that allow secure communication, quantum computers that outperform supercomputers, and quantum simulators of complex physical problems inaccessible to solve with current computing technologies. In order for this to happen, we need to efficiently produce particles, control their states, detect them and make them interact strongly with each other. Photons, quantum particles of light, are one of the most promising building blocks of future quantum technologies. We can easily detect and control their states and we can efficiently produce them individually. However, making them interact strongly to build a large quantum network is a notoriously difficult task because photons do not interact at low energies. To make them interact indirectly, we can hybridise them with other particles that do strongly interact and form new particles called 'polaritons'.

In this project, we aim to hybridise photons with Rydberg excitons. Rydberg excitons are highly excited electron-hole pairs that can span macroscopic dimensions. Because of their macroscopic dimensions they strongly repel each other. The semiconductor device that we have chosen for hybridisation is a 2-dimensional semiconductor microcavity formed by two highly reflective mirrors encompassing nanocrystals and thin films of cuprous oxide. Photons confined in the microcavity strongly couple to Rydberg excitons in cuprous oxide to form Rydberg polaritons. The Rydberg polaritons interaction strength will be orders of magnitude higher than the current microcavity polaritons. This breakthrough will allow us to explore quantum optics at the single-particle limit and form 2-dimensional networks of strongly correlated photons for future single-photon switches and quantum simulators.

The development of single-photon nonlinearity in semiconductor microcavities with their proven robustness and potential for scalability as proposed in this research project offers a new paradigm to quantum technologies with photons. Our proposal will develop new physics that in the short term will be of major interest to a broad range of academics working in fundamental science, and in the medium and long term will feed into new advanced optoelectronic devices which would be of major interest to the multibillion-pound quantum technologies industry.

Impact on academics and researchers working on nonlinear photonics, topological photonics, quantum optics including single-photon sources/switches/transistors, and quantum simulations will be via traditional methods such as journal publications, conference presentations, seminars, and workshops. We will also organise a workshop for PhD students and early career postdocs with 5 experienced professors working in light-matter interaction. The workshop will provide a friendly atmosphere for the scientific exchange of ideas for every participant and also facilitate the sharing of personal experience and expertise of the senior PIs to improve on the transferable skills of the next generation of scientists in the UK.

The research area in this project combines semiconductor physics, advanced optical spectroscopy, quantum physics, cryogenics and fabrication, and therefore provides an ideal training ground for researchers (through Condensed Matter and Photonics centres for doctoral training) to take these skills into UK employment. Allowing both the PhD students and postdocs to gain experience in fabrication and working with the industrial partner will give them an excellent vision of the potential difficulties of taking science into the market, as well as creating a large network of contacts for them.

In the medium term, the outcomes are more likely to impact the exploitation of this new physics for commercial devices by miniaturisation, room-temperature operation, and integration into well-developed platforms such as silicon photonics, and fibre communications. This will be through major photonics companies like Thorlabs, with whom I have previously had a successful knowledge transfer partnership. One key impact of this grant is to develop further core relationships with companies who can engage in long-term partnerships. I have past experience with Hitachi, and they are keen to be involved in the current work because of its overlap with their interests. Hitachi has wide experience in quantum emitters, and will be involved in knowledge transfer. This is an ideal way to combine the industrial experience of Hitachi, which includes the intensive development of quantum technologies with the fundamental studies proposed in this project.