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arXiv:2509.02392 · Optics Express

Widely non-degenerate nonlinear frequency conversion in cryogenic titanium in-diffused lithium niobate waveguides

Nina Amelie Lange, Sebastian Lengeling, Martin Philipp Mues, Viktor Quiring, Werner Ridder, Christof Eigner, Harald Herrmann, Christine Silberhorn, Tim J. Bartley

Published Oct 20, 2025• 2 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/oe.578108

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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.
We demonstrate the generation of degenerate photon pairs from spontaneous parametric down-conversion in titanium-indiffused waveguides in lithium niobate at cryogenic temperatures. Since the phase matching cannot be temperature tuned inside a cryostat, we rely on a precise empirical model of the refractive indices when fabricating a fixed poling period. We design the phase-matching properties of our periodic poling to enable signal and idler photons at 1559.3(6)1559.3(6)1559.3(6) nm and characterize the indistinguishability of our photons by performing a Hong-Ou-Mandel interference measurement. Despite the effects of photorefraction and pyroelectricity, which can locally alter the phase matching, we achieve cryogenic indistinguishable photons within 1.5 nm of our design wavelength. Our results verify sufficient understanding and control of the cryogenic nonlinear process, which has wider implications when combining quasi-phase-matched nonlinear optical processes with other cryogenic photonic quantum technologies, such as superconducting detectors.
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] .
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.
Dataset of the publication "Degenerate photons from a cryogenic spontaneous parametric down-conversion source", N. A. Lange, T. Schapeler, J. P. Höpker, M. Protte, and T. J. Bartley, https://doi.org/10.48550/arXiv.2303.17428 (2023). Abstract: We demonstrate the generation of degenerate photon pairs from spontaneous parametric down-conversion in titanium-indiffused waveguides in lithium niobate at cryogenic temperatures. Since the phase matching cannot be temperature tuned inside a cryostat, we rely on a precise empirical model of the refractive indices when fabricating a fixed poling period. We design the phase-matching properties of our periodic poling to enable signal and idler photons at 1559.3(6) nm and characterize the indistinguishability of our photons by performing a Hong-Ou-Mandel interference measurement. Despite the effects of photorefraction and pyroelectricity, which can locally alter the phase matching, we achieve cryogenic indistinguishable photons within 1.5 nm of our design wavelength. Our results verify sufficient understanding and control of the cryogenic nonlinear process, which has wider implications when combining quasi-phase-matched nonlinear optical processes with other cryogenic photonic quantum technologies, such as superconducting detectors.
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.