Integrated quantum photonic devices require the combination of quantum light sources, elements for guiding and manipulating the photonic states and detectors [1] . In a fully-integrated platform, the operating conditions for each component must be mutually compatible. Whilst parametric down-conversion (PDC) as a source for photonic states is well-established under ambient conditions [2] , other quantum photonic technologies require cryogenic operation temperatures, in particular superconducting single-photon detectors [3] .
Integrated quantum photonic devices require the combination of quantum light sources, elements for guiding and manipulating the photonic states and detectors [1] . In a fully-integrated platform, the operating conditions for each component must be mutually compatible. Whilst parametric down-conversion (PDC) as a source for photonic states is well-established under ambient conditions [2] , other quantum photonic technologies require cryogenic operation temperatures, in particular superconducting single-photon detectors [3] .
The realisation of light modulation based on single photon detection events is an important step towards feed-forward-modulation, which is required for many quantum optics protocols. This requires combining detection events with modulation in a device with a small physical footprint, for which integrated optics is ideal. Several platforms for integrated quantum optics have been developed to generate, modulate, and detect quantum features in the light over the last years. Titanium in-diffused waveguides in lithium niobate is a promising candidate for quantum communication applications [1] , [2] , however, single photon detection with superconducting photon detectors (SNSPDs) require cryogenic temperatures. Therefore, optimising the nonlinear optical properties of these waveguides under cryogenic operating conditions is an important task. Previously, we have demonstrated frequency conversion [3] , electro-optical modulation [4] and integrated single photon detection [5] at cryogenic temperatures.
Abstract Lithium niobate is a promising platform for integrated quantum optics. In this platform, we aim to efficiently manipulate and detect quantum states by combining superconducting single photon detectors and modulators. The cryogenic operation of a superconducting single photon detector dictates the optimisation of the electro-optic modulators under the same operating conditions. To that end, we characterise a phase modulator, directional coupler, and polarisation converter at both ambient and cryogenic temperatures. The operation voltage V π / 2 of these modulators increases, due to the decrease in the electro-optic effect, by 74% for the phase modulator, 84% for the directional coupler and 35% for the polarisation converter below 8.5 K . The phase modulator preserves its broadband nature and modulates light in the characterised wavelength range. The unbiased bar state of the directional coupler changed by a wavelength shift of 85 n m while cooling the device down to 5 K . The polarisation converter uses periodic poling to phasematch the two orthogonal polarisations. The phasematched wavelength of the utilised poling changes by 112 n m when cooling to 5 K .
The titanium in-diffused lithium niobate waveguide platform is well-established for reliable prototyping and packaging of many quantum photonic components at room temperature. Nevertheless, compatibility with certain quantum light sources and superconducting detectors requires operation under cryogenic conditions. We characterize alterations in phase-matching and mode guiding of a non-degenerate spontaneous parametric down-conversion process emitting around 1556 nm and 950 nm, under cryogenic conditions. Despite the effects of pyroelectricity and photorefraction, the spectral properties match our theoretical model. Nevertheless, these effects cause small but significant variations within and between cooling cycles. These measurements provide a first benchmark against which other nonlinear photonic integration platforms, such as thin-film lithium niobate, can be compared.
Scalable quantum photonics relies on interfacing many optical components under mutually compatible operating conditions. To that end, we demonstrate that spontaneous parametric down-conversion (SPDC) in nonlinear waveguides, a standard technology for generating entangled photon pairs, squeezed states, and heralded single photons, is fully compatible with cryogenic operating conditions required for superconducting detectors. This is necessary for the proliferation of integrated quantum photonics in integration platforms exploiting quasi-phase-matched second-order nonlinear interactions. We investigate how cryogenic operation at 4 K affects the SPDC process by comparing the heralding efficiency, second-order correlation function, and spectral properties with operation at room temperature.
Superconducting detectors are now well-established tools for low-light optics, and in particular quantum optics, boasting high-eciency, fast response and low noise. Similarly, lithium niobate is an important platform for integrated optics given its high second-order nonlinearity, used for high-speed electro-optic modulation and polarization conversion, as well as frequency conversion and sources of quantum light. Combining these technologies addresses the requirements for a single platform capable of generating, manipulating and measuring quantum light in many degrees of freedom, in a compact and potentially scalable manner. We will report on progress integrating tungsten transition-edge sensors (TESs) and amorphous tungsten silicide superconducting nanowire single-photon detectors (SNSPDs) on titanium in-diused lithium niobate waveguides. e travelling-wave design couples the evanescent eld from the waveguides into the superconducting absorber. We will report on simulations and measurements of the absorption, which we can characterize at room temperature prior to cooling down the devices. Independently, we show how the detectors respond to ood illumination, normally incident on the devices, demonstrating their functionality.
Photonic integrated circuits operating at cryogenic temperatures are necessary for many quantum technologies such as quantum transduction, integrated single-photon emitters and detectors, as well as deep-space communication and sensing devices. Thin-film lithium niobate (TFLN) is an emerging platform that is a strong candidate for fully integrated quantum photonics, offering low loss, fast electro-optic reconfigurability, nonlinear quantum light sources, and the ability to host quantum emitters and single-photon detectors. To interface TFLN with technologies that require cryogenic operation, like superconducting single-photon detectors, microwave-to-optical transducers, and solid-state quantum emitters, it is important to study its optical and electrical properties from room temperature down to cryogenic temperatures. Here, we investigate linear and nonlinear photonic devices, including racetrack resonators, Mach-Zehnder modulators and periodically poled waveguides in TFLN using a cryogenic fiber probe station with full temperature control down to 5 K. We quantify a shift in resonances, a 22% increase in electro-optic modulator half-wave voltage, a blue shift of 18 nm for Type-0 phase-matching as well as a red shift of 64 nm for Type-II phase-matching as the sample temperature decreases. Our study of nonlinear processes in a cryogenic environment will contribute towards developing novel devices for inter-platform quantum information processing, secure communication, and enhanced sensing.