Enhancement of single-photon purity and brightness via bound states in the continuum

The photon blockade is a promising mechanism for single-photon sources. However, in most systems with photon blockade, the brightness of the quantum source (being proportional to the average photon number in cavities) is limited. A recent experiment [Lei et al., Phys. Rev. Lett. 130, 093801 (2023)] proves that Friedrich-Wintgen bound states in the continuum (BICs) arise when a microresonator containing a pair of near-resonant modes is side-coupled to an adjacent bus waveguide where the gap between them is narrow. To our knowledge, the impacts of BICs on the single-photon purity and brightness in the blockade process remain less explored. Here, we put forward a scheme to simultaneously enhance both the purity and brightness of single-photon emission in one cavity mode of such an integrated microresonator-waveguide system, while the other cavity mode is considered to be an active mode, modeled as incoherent-pumping (gain)-like laser production. Physically, this enhancement is achieved through BICs produced by quantum destructive interference between different dissipative pathways. We demonstrate that BICs enable the improvement of emission brightness by 1 to 2 orders of magnitude, accompanied by high single-photon purity of 0.98 (or strong photon blockade of 0.02) compared with the BIC-free configuration. As a check, the full numerical results for the second-order correlation function and the average photon number of the emission cavity mode, which respectively determine the purity and brightness of single-photon emission, are in excellent agreement with the semianalytical solutions. Our BICs-based work offers a promising avenue for constructing single-photon sources with high purity and high brightness.