Inline Quantum Measurements with SNSPDs Coupled to Photonic Bound States

Superconducting nanowire single-photon detectors (SNSPDs) are enabling components of quantum photonic integrated circuits for their ease of fabrication and unsurpassed performance. While several approaches for SNSPDs integration are being pursued, scalability and interfacing with electrical readout circuits remain challenging. Here we introduce the concept of inline detection with SNSPDs, facilitating advanced quantum measurements within an ultra-compact device footprint. To establish this approach, we develop a photonic bound states in the continuum (BIC) platform based on etchless polymer waveguides, which substantially suppress parasitic scattering at detector terminations while ensuring compatibility with standard photonic substrates and cryogenic operation. We show BIC-coupled inline detectors with on-chip efficiency exceeding 80%, recovery time of less than 2 ns, and intrinsic jitter of less than 70 ps. As a proof of principle, we implement a Hanbury Brown and Twiss interferometer with footprint of 60x6 um2, and a photon number resolving detector for discrimination of collinear photon pairs. The demonstration of accurate and reliable inline quantum measurements within a simple and scalable photonic architecture offers a viable pathway to realize more complex quantum circuit functionalities, such as higher-order correlation measurements, quantum state tomography, and multi-photon subtraction.