Single-photon detection and cryogenic reconfigurability in Lithium Niobate nanophotonic circuits.

Lithium-Niobate-On-Insulator (LNOI) is emerging as a promising platform for integrated quantum photonic technologies because of its high second-order nonlinearity, small waveguide footprint, and ultra-low propagation loss in a broad wavelength range. Importantly, LNOI allows for creating electro-optically reconfigurable circuits, which can be efficiently operated at cryogenic temperature. Their integration with superconducting nanowire single-photon detectors (SNSPDs) paves the way for realizing scalable photonic devices for active manipulation and detection of quantum states of light. Here we report the first realization of these two essential components in a single LNOI photonic circuit. As an experimental showcase of our technology, we demonstrate the combined operation of an electrically tunable Mach-Zehnder interferometer - a fundamental building block for the realization of reconfigurable optical networks - and two waveguide-integrated SNSPDs at its outputs. Our results provide blueprints for implementing complex quantum photonic devices on a LNOI platform.