  We demonstrate a fiber-integrated quantum optical circulator that is operated
by a single atom and that relies on the chiral interaction between emitters and
transversally confined light. Like its counterparts in classical optics, our
circulator exhibits an inherent asymmetry between light propagation in the
forward and the backward direction. However, rather than a magnetic field or a
temporal modulation, it is the internal quantum state of the atom that controls
the operation direction of the circulator. This working principle is compatible
with preparing the circulator in a coherent superposition of its operational
states. Such a quantum circulator may thus become a key element for routing and
processing quantum information in scalable integrated optical circuits.
Moreover, it features a strongly nonlinear response at the single-photon level,
thereby enabling, e.g., photon number-dependent routing and novel quantum
simulation protocols.
