Integrated quantum photonics has rapidly developed in recent years, as it holds great promise for realizing complicated quantum photonic applications with small footprints, therefore overcoming the limitations of traditional bulk optics. Single-photon detectors are essential components of quantum photonic systems, making the on-chip integration of these detectors a pressing requirement that can enhance scalability and eliminate lossy fiber interconnections. With outstanding performance, superconducting nanowire single-photon detectors (SNSPDs) are widely utilized in quantum photonic experiments, exhibiting ultrahigh detection efficiency, negligible dark count rate and low timing jitter, which makes them among the most attractive candidates for co-integration with quantum photonic circuits.

Suppression of photonic noise is necessary for SNSPDs to be properly operating, particularly when the pump power is substantially higher than the generated photon pairs. In most experiments related with photon pair generation, pump rejection is achieved by off-chip filters before detection, thereby hindering the compactness of the whole system. On-chip pump filters have been demonstrated through various strategies on silicon photonics platform, such as cascaded microrings , cascaded Mach–Zehnder interferometers, and contra-directional couplers , and some of them are monolithically integrated with photon pair sources as entanglement suppliers. However, in future large-scale quantum networks, the receivers may require pump filters with different wavelengths and rejection ratios for various applications; thus it would be more flexible to integrate different pump filters on receiver side, allowing for unique demands. But when it comes to entanglement receivers, integrating pump filters with SNSPDs on a single chip still remains challenging due to the stringent requirements of cryogenic environments, which limit the use of active tuning elements. Silicon Bragg grating filters are entirely passive, making them cryogenic-compatible. Moreover, their robustness to fabrication imperfections allows for cascading multiple sections to obtain a high pump rejection ratio and provides tolerance to additional fabrication process induced by SNSPDs.

In this work, we demonstrate the heterogeneous integration of SNSPDs and pump rejection filters (PRFs) on silicon photonics platform, with the aim of removing strong pump light prior to entanglement characterization. Our on-chip SNSPDs exhibit saturated detection efficiency, and by cascading seven sections of Bragg grating filters, we achieve on-chip pump rejection exceeding 56 dB. This integration combines the advantages of high-performance SNSPDs and PRFs on a single chip, eliminating the need for fiber or bulk optical filters and the associated separate interfaces. In addition, by feeding correlated photon pairs from a silicon microring resonator, we proceed to analyze energy-time entanglement using a dense wavelength division multiplexer (DWDM) and Franson-type interferometers, showing two-photon interference fringes with visibilities of 92.85%±5.95% and 91.91%±7.34%, along with a coincidence-to-accidental ratio (CAR) that reaches 32. Our results effectively show the successful removal of pump light before detection, bringing a step closer to future scalable quantum information processing.