Integrated nonreciprocal optical components, which have an inherent asymmetry between
their forward and backward propagation direction, are key for routing signals in photonic
circuits. Here, we demonstrate a fiber-integrated quantum optical circulator operated by a
single atom. Its nonreciprocal behavior arises from the chiral interaction between the atom
and the transversally confined light. We demonstrate that the internal quantum state of the
atom controls the operation direction of the circulator and that it features a strongly nonlinear …

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 …