A traffic jam of light opens a path to realizing previously unseen exotic phases of matter

When light passes through matter, it slows down. Light can even be brought to a standstill when it travels through carefully designed matter. One way in which this occurs is when the velocity of individual particles of light (photons) in a material is zero. Another, more intriguing, way is when photons, which normally pass through each other unimpeded, are made to repel each other. If the repulsion is strong enough, the photons are unable to move, and the light is frozen in place. Writing in Nature, Ma et al.¹ report that such a phase of matter, known as a Mott insulator, can be produced by exploiting energy loss in a system in which photons move through an array of superconducting circuits.