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 and compact waveguide footprint. 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 demonstrate integration of these two key components in a low loss (0.2 dB/cm) LNOI waveguide network. As an experimental showcase of our technology, we demonstrate the combined operation of an electrically tunable Mach-Zehnder interferometer and two waveguide-integrated SNSPDs at its outputs. We show static reconfigurability of our system with a bias-drift-free operation over a time of 12 hours, as well as high-speed modulation at a frequency up to 1 GHz. Our results provide blueprints for implementing complex quantum photonic devices on the LNOI platform.

Heterogeneously Integrated Photonic Chip on Lithium Niobate Thin-Film Waveguide

Lithium niobate thin film represents as an ideal material substrate for quantum photonics due to its strong electro-optic effect and high-speed modulation capability. Here, we propose a novel platform which heterogeneously integrates single self-assembled InAs/GaAs quantum dots for a single-photon source on a lithium niobate photonic chip. The InAs/GaAs quantum dots can be transferred to the lithium niobate waveguide via a substrate transfer procedure with nanometer precision and be integrated through van der Waals force. A down-tapered structure is designed and optimized to deliver the photon flux generated from the InAs quantum dots embedded in a GaAs waveguide to the lithium niobate waveguide with an overall efficiency of 42%. In addition, the electro-optical effect is used to tune, and therefore to tune the beam splitting ratio of the integrated lithium niobate directional coupler, which can simultaneously route multiple photons to different spatial modes, and subsequently fan out through grating couplers to achieve single-photon sub-multiplexing. The proposed device opens up novel opportunities for achieving multifunctional hybrid integrated photonic chips.