Figure 1
APS/Joan Tycko

Figure 1: A photon transistor uses a light source to modulate a medium’s opacity to subsequent photons. To make a medium whose opacity can be switched with a single photon, researchers used highly excited atoms in an atom cloud. When an incoming photon (​red) excites an atom to a Rydberg state (red​ sphere​), ​the excited atom perturbs the atoms within a certain “blockade radius” (purple volume) rendering them either opaque or transparent to a subsequent light source.