Chiral topological add–drop filter for integrated quantum photonic circuits

The integration of quantum emitters within topological nano-photonic devices opens up new avenues for the control of light-matter interactions at the single photon level. Here, we experimentally realise an integrated topological add-drop filter and observe multi-port chiral emission from single photon emitters (quantum dots) embedded within the device. The filter is imprinted within a valley-Hall photonic crystal (PhC) membrane and comprises a resonator evanescently coupled to a pair of access waveguides. We show that the longitudinal modes of the resonator enable the filter to perform wavelength-selective routing of light, protected by the underlying topology. Furthermore, we demonstrate that for a quantum dot located at a chiral point in the resonator, selective coupling occurs between well-defined spin states and specific pairs of output ports of the filter. The combination of multi-port routing, allied with the inherent non-reciprocity of the device at the single photon level, presents opportunities for the formation of complex quantum optical devices, such as an on-chip quantum optical circulator.