We report the realization of inline quantum measurements with waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs). To suppress parasitic scattering at detector terminations, while ensuring compatibility with standard photonic substrates and cryogenic operation, we developed a photonic bound states in the continuum (BIC) platform based on etchless polymer waveguides. 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 Hanbury Brown and Twiss interferometry and photon number resolution with two collinear detectors within a footprint of 60⋅6μm2. The concept of inline quantum measurements could be further developed to support more complex circuit functionalities, such as higher-order correlation measurements, quantum state tomography, and multi-photon subtraction, within a compact architecture.
@article{arxiv.2506.07029,
title = {Inline Quantum Measurements with SNSPDs Coupled to Photonic Bound States},
author = {Filippo Martinelli and Anton N. Vetlugin and Shuyu Dong and Darren M. Z. Koh and Mariia Sidorova and Christian Kurtsiefer and Cesare Soci},
journal= {arXiv preprint arXiv:2506.07029},
year = {2025}
}