Cryogenic-compatible ultrahigh rejection and narrowband sinusoidal Bragg filter for integrated quantum photonics

The cryogenic environment offers a promising platform for integrated quantum photonic systems thanks to its low noise characteristics and compatibility with key quantum components, such as superconducting detectors and quantum dots. To harness the full potential of cryogenic integrated quantum photonic circuits, the development of cryogenic adapted on-chip optical filters with exceptional performance is crucial, particularly for efficient extraction of single photon signals from intense pump light in nonlinear processes like spontaneous four-wave mixing. In this work, we demonstrate a cryogenic-compatible Bragg filter, featuring a sinusoidal coherency-broken cascaded architecture for ultrahigh rejection and narrowband. Two subwavelength gratings are deployed to eliminate the residual transverse magnetic mode photons induced by imperfect polarization alignment. Furthermore, a niobium nitride film is deposited on the chip surface as a light absorption layer to suppress background noise caused by scattered photons. The fabricated filter exhibits a rejection ratio of 82 dB at room temperature, and maintains 76 dB at a cryogenic temperature of 2.2 K, with corresponding bandwidths of 0.9 nm and 0.8 nm, respectively. These results confirm the suitability of this filter for the cryogenic working environment, providing crucial support for the cryogenic-compatible integrated quantum photonic circuits.