Theory reveals the shape of a single photon

Research at the University of Birmingham, published in Physical Review Letters, explores the nature of photons (individual particles of light) in unprecedented detail to show how they are emitted by atoms or molecules and shaped by their environment.

The nature of this interaction leads to infinite possibilities for light to exist and propagate, or travel, through its surrounding environment. This limitless possibility, however, makes the interactions exceptionally hard to model, and is a challenge that quantum physicists have been working to address for several decades.

By grouping these possibilities into distinct sets, the Birmingham team were able to produce a model that describes not only the interactions between the photon and the emitter, but also how the energy from that interaction travels into the distant ‘far field’.

At the same time, they were able to use their calculations to produce a visualisation of the photon itself.

First author Dr Benjamin Yuen, in the University’s School of Physics and Astronomy, explained: “Our calculations enabled us to convert a seemingly insolvable problem into something that can be computed. And, almost as a bi-product of the model, we were able to produce this image of a photon, something that hasn’t been seen before in physics.”

The work is important because it opens up new avenues of research for quantum physicists and material science. By being able to precisely define how a photon interacts with matter and with other elements of its environment, scientists can design new nanophotonic technologies that could change the way we communicate securely, detect pathogens or control chemical reactions at a molecular level, for example.

Co-author Professor Angela Demetriadou, also at the University of Birmingham, said: “The geometry and optical properties of the environment has profound consequences for how photons are emitted, including defining the photon’s shape, colour, and even how likely it is to exist.”

Yuen added: “This work helps us to increase our understanding of the energy exchange between light and matter, and secondly, to better understand how light radiates into its nearby and distant surroundings. Lots of this information had previously been thought of as just ‘noise’ — but there’s so much information within it that we can now make sense of, and make use of. By understanding this, we set the foundations to be able to engineer light–matter interactions for future applications, such as better sensors, improved photovoltaic energy cells, or quantum computing.”

Image credit: Dr Benjamin Yuen