The polariton, or exciton polariton, to give it its full name. How can such a particle be light and matter simultaneously? As is so often the case, the secret lies in the weirdness of quantum physics. Polaritons are not the kind of beasts you can expect to see in the wild. First bred in 1991 by researchers at the University of Tokyo, Japan, they spend the entirety of their short lives in tiny mirrored cages known as semiconductor microcavities.

The gestation of polaritons is a complex process. It begins in a sandwich of semiconducting materials known as a quantum well. Electrons are jammed tightly into the thin, sheet-like filling of this sandwich - typically less than a micrometre thick - and so are particularly excitable. Add a little drop of energy, in the form of light or a voltage, and some of the electrons absorb it and jump to a higher energy level, leaving behind an absence of electrons - positively-charged "holes". An electron-and-hole pairing is called an exciton, and is usually a short-lived affair: the energised electron soon gives up its extra energy and plonks itself back into the hole. At the same time it releases the energy it had taken on board, in the form of a photon of light.