Superconducting nanowire single-photon detectors with saturated quantum efficiency at 1550 nm on single-crystal diamond substrates

Single-crystal diamond possesses exceptional physical and optical properties, rendering it an ideal platform for integrated quantum optics. The direct integration of broadband-sensitive and high-performance single-photon detectors on diamond holds significant implications for the realization of integrated diamond quantum optical circuits. In this study, we polished the diamond surface with RMS (root mean square) below 0.6 nm suitable for the deposition and patterning of NbN thin films through ion beam etching. Subsequently, we fabricated superconducting nanowire single-photon detectors directly on the polished diamond substrates and characterized for their electrical and optical properties. The NbN-SNSPD exhibited a high critical current density (2 MA cm-2), a saturated quantum efficiency (QE) below 2.5 K, and a maximum value of QE up to 88% at 4 K. These findings offer a promising solution for fully integrated quantum optical chips on single-crystal diamond substrates.