Chiral exceptional bound states in the continuum: A higher-order singularity for on-chip control of quantum emission

We demonstrate a fully integrable and reconfigurable platform for controlling quantum emission by harnessing chiral exceptional bound states in the continuum (BICs) as a higher-order non-Hermitian singularity. Our architecture employs dual-microring resonators evanescently coupled to two waveguides, supporting symmetry-protected BICs. By integrating a waveguide-coupled reflector coupled with one resonator as a unidirectional feedback, a pair of orthogonal BICs gets transformed into a single, chiral quasi-BIC residing on an exceptional surface. The phase terms in external coupling and intermodal coupling serve as two independent tuning knobs, enabling unprecedented dynamic control over the spontaneous emission dynamics of individual quantum emitters, including the Purcell enhancement and the emission line shape. The efficiency in reconfiguring the output intensity gets promoted by more than a factor of two compared to alternative schemes, offering a promising path toward high-speed quantum optical switches and active lifetime control in integrated quantum photonic circuits.