Quantum optical circulator controlled by a single chirally coupled atom

A quantum optical circulator A circulator is a passive three- or four-port device that routes signals according to a simple protocol: If the ports are numbered in ascending order, a signal that enters the circulator through port 1, 2, 3, or 4 exits it through port 2, 3, 4, or 1, respectively. Scheucher et al. demonstrate an integrated optical circulator that operates by using the internal quantum state of a single atom (see the Perspective by Munro and Nemoto). Moreover, the routing can be reversed by flipping the atomic spin. Such an integrated optical device may be important for routing and processing quantum information in scalable integrated optical circuits. Science, this issue p. 1577; see also p. 1532 The internal state of a single atom is used to route single photons in an optical circulator. 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 response at the single-photon level. This enables, for example, photon number–dependent routing and novel quantum simulation protocols. Furthermore, such a circulator can in principle be prepared in a coherent superposition of its operational states and may become a key element for quantum information processing in scalable integrated optical circuits.