A new theory, that explains how light and matter interact at the quantum level has enabled researchers to define the precise 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...