Bose-Einstein Condensation of Photons

This proposal is to make and study a novel, collective, quantum state of light by using a dye medium between two mirrors to bring photons to thermal equilibrium. The collective state emerges because of quantum interference.

Light is made of photons. Photons can be absorbed by a fluorescent dye, and bounced between two mirrors just 2 millionths of a metre apart, for one billionth of a second. That's more than long enough for the photons to come into thermal equilibrium with the dye, at room temperature. Quantum mechanics makes sure that identical photons interfere constructively, and so are very likely to be found together. This means that if we shine a bright enough light on our dye-between-mirrors, we will create a giant quantum wave, known as a Bose-Einstein condensate (BEC).

BECs have been seen in laser-cooled atoms in vacuum chambers, in cryogenic helium, and in semiconductor systems (exciton-polaritons) at low temperatures. In 2010, BEC was seen for the first time at room temperature in a dye-between-mirrors. This project will take that simple experimental idea and use it to study the properties of photon BECs.

The mirrors will be curved to confine the light: the shape of the mirrors gives an effective potential energy. Part of this project will be developing a machine and technique to make custom-shaped mirrors to confine photons so that they can only move in one dimension. Another part will be looking at the coherence of the photon BEC (how it shows interference). The photons in the dye medium interact with each other, unlike light beams in free space. I will study those photon interactions. I will also look at how the photon BEC behaves away from thermal equilibrium. The end result of this project will be an understanding of how photon BECs form, what are their properties, and their interactions, as well as the ability to make them in arbitrary sizes and shapes.

This is a fundamentally new way to manipulate light, so we can expect new optical devices will follow. It is hoped that this system of thermalised photons will improve the cost-efficiency of solar energy conversion, by transforming the frequency and spatial mode of the light that is pumped into the dye, making it possible to use optimised, cheap photovoltaics.

This project will be working with a new and very unusual state of light, created by trapping photons and allowing them to come to thermal equilibrium in a fluorescent dye. In this Bose-Einstein condensate of photons, quantum mechanical effects are visible on human scales. Depending on the experimental results, we will know whether it is possible to use the device I will build to create a fixed photon-number state, and how to use it to improve the cost efficiency of solar energy harvesting. The device will be rather simple, and could straightforwardly be made robust which makes it ideal either for commercial applications or for demonstrations to the general public.

The main potential economic application of this work involves solar energy harvesting. Fluorescent solar collector technology can make cheaper solar cells than standard photovoltaic devices by taking sunlight from a large area and concentrating it around an edge, where smaller (cheaper) photovoltaics can be used for power generation. In this project, the dye plays the same fluorescence role, but the mirrors also spatially separate the different wavelengths of light. This means that an array of wavelength-optimised photovoltaics could be used, which would boost the efficiency of energy conversion. Thus, this project may well lead to more efficient solar cells, adding to the technologies available for renewable energy.

This work will show whether it is possible to created fixed-number states of light. These states can be used in quantum metrology to maximise the sensitivity of measurement when working with fragile objects (e.g. biological samples), using as few photons as possible.

Another integral part of this project is its outreach programme. The main apparatus of the experiment can be made very simple and robust. The observation of the system is done with a simple microscope and the photon Bose-Einstein condensate comes out as visible light. It may even be possible to see this quantum object directly with the human eye. This makes the apparatus ideal for public demonstration at exhibitions, science fairs and schools, showing non-scientific audience what cutting-edge science looks like.