Quantum optical circulator controlled by a single chirally coupled atom

Here we demonstrate the experimental realization of a fiber-integrated optical circulator whose operation direction is controlled by the quantum state of a single atom. The device is based on a bottle whispering-gallery-mode microresonator interfaced by two optical nanofibers. Depending on its internal state, the single atom couples differently to the chiral resonator fields. This results in a direction dependent transmission through the resonator and thus realizes an optical circulator. The demonstrated quantum optical circulator opens new ways for routing and processing quantum information in optical circuits.