Understanding Bose-Einstein Condensation of Light

The search for quantum effects on macroscopic scales has fascinated many physicists over the past century. Bose-Einstein condensation (BEC) is one route to such remarkable behaviour. BEC occurs in systems formed from large collections of bosons (particles which follow Bose-Einstein statistics), such as photons (the quantised particle of light), and atoms with an even number of constituent parts. A BEC forms when the temperature of a gas becomes so low that the only way the energy distribution can follow the rules of quantum mechanics is by transitioning to a phase in which many of the particles are in the single lowest energy quantum state. This causes a dramatic change in the properties of the gas as the whole ensemble behaves as a single quantum particle.

Initially it was thought that a BEC of photons could never form since the number of particles is not conserved, and so as a gas of photons is cooled the particles are absorbed by the container, and the gas does not condense. Remarkably, in 2010, around 100 years since the relevant physics was first discussed, this problem was overcome and the room temperature BEC of photons was experimentally observed. This success has opened up a whole new set of questions which must be addressed in order to maximise the potential for future experiments. These systems, while sharing many similarities with the conventional equilibrium BECs observed in ultra-cold atoms, are very different due to the finite lifetime of the particles. This gives rise to a rich variety of physical behaviour including non-equilibrium superfluidity, spontaneous quantised vortices and other exotic phenomena which I intend to explore.

My previous work has addressed the latest experimental results. However, there is significant interest in using these systems as a toolbox for understanding many-body quantum phenomena (complex behaviour requiring many interacting particles). For these applications the tools developed so far are inadequate. Systems such as these, with strong coupling and competition between coherent quantum effects and losses, are very challenging to treat, analytically or numerically. The tools I propose to develop here will provide a route to understanding the behaviour of these room temperature quantum coherent systems.

This project aims to understand the behaviour of two new classes of non-equilibrium quantum condensate: the photon condensate and the organic polariton condensate. These systems have generated much recent experimental interest since they are able to display quantum coherent properties at room temperature. This proposal aims to provide a comprehensive theoretical model of these systems which will enable us to firstly understand the physical principles governing these devices and then to predict the necessary system properties necessary to observe interesting and complex behaviour.

The impacts of this research will mainly be in interacting with other scientists studying related fields both experimentally and theoretically. There is an increasing number of experimental groups studying the systems described in this proposal around the world and the work performed here will be disseminated to them via both publications in internationally recognised journals and presentations at conferences. I will also directly collaborate with a number of these groups, especially the group of Dr. Nyman at Imperial College who is a partner in this project.

As part of the project I also plan to arrange a short workshop, with approximately thirty delegates which will bring together researchers with expertise in different but closely related fields which are all closely tied to this project.

Some of the research will require me to write computer code to perform simulations. This code will also be of use to researchers in other related fields and so will be fully documented and made available as an open source project. This will allow others to both use and contribute to the code produced.

There are many potential long term applications for these systems, they may find uses in the development of highly efficient solar cells and they may be able to produce laser-like light in frequency regimes where this not currently possible such as UV where they could be used for high resolution lithography.