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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Non-phase-matched spontaneous parametric down-conversion from lithium niobate thin films

Ngoc My Hanh Duong, Andreas Maeder, Grégoire Saerens, Fabian Kaufmann, Rachel Grange

Published Jun 21, 2021• 1 citation
Publisher

Abstract

Spontaneous parametric down-conversion (SPDC) is one of the most efficient sources for quantum emitters, which holds promise for numerous applications in quantum key distribution [1] , quantum computing [2] , and quantum networks [3] . Despite its vast potential yet simple measurement setup, the applicability of the SPDC sources is still limited due to their modest conversion efficiency. Therefore, many attempts have been made to improve the efficiency of the process, but mainly focused on phase-matched regime which relied upon the use of macroscopic birefringent crystals or material engineering. Here, we study the SPDC process at the telecom wavelength range in 200 nm thick thin-film lithium niobate (TFLN). This study not only provides an insight into the two-photon light generation process in the new non-phase-matched regime [4] , [5] also paves the way for the monolithic integration of available SPDC sources in chip-based devices [6] and metasurfaces [7] from nonlinear materials such as barium titanate, and gallium arsenide.

DOI
10.1109/cleo/europe-eqec52157.2021.9542676

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Spontaneous parametric down-conversion (SPDC) is one of the most efficient sources for quantum emitters, which holds promise for numerous applications in quantum key distribution [1] , quantum computing [2] , and quantum networks [3] . Despite its vast potential yet simple measurement setup, the applicability of the SPDC sources is still limited due to their modest conversion efficiency. Therefore, many attempts have been made to improve the efficiency of the process, but mainly focused on phase-matched regime which relied upon the use of macroscopic birefringent crystals or material engineering. Here, we study the SPDC process at the telecom wavelength range in 200 nm thick thin-film lithium niobate (TFLN). This study not only provides an insight into the two-photon light generation process in the new non-phase-matched regime [4] , [5] also paves the way for the monolithic integration of available SPDC sources in chip-based devices [6] and metasurfaces [7] from nonlinear materials such as barium titanate, and gallium arsenide.
Spontaneous parametric down-conversion (SPDC) has become a key method for generating entangled photon pairs. Periodically poled thin-film lithium niobate (TFLN) waveguides induce strong SPDC but require complex fabrication processes. In this work, we experimentally demonstrate efficient SPDC and second harmonic generation using modal phase matching methods. This is achieved with inverse-designed optical mode converters and low-loss optical waveguides in a single nanofabrication process. Inverse design methods provide enhanced functionalities and compact footprints for the converter. Despite the extensive achievements in inverse-designed photonic integrated circuits, the potential of inverse-designed TFLN quantum photonic devices has been seldom explored. The device shows an on-chip conversion efficiency of 3.95% W−1 cm−2 in second harmonic generation measurements and a coincidence count rate up to 21.2 kHz in SPDC experiments. This work highlights the potential of the inverse-designed TFLN photonic devices and paves the way for their applications in on-chip nonlinear or quantum optics.
State-of-the-art photon sources based on spontaneous parametric down-conversion (SPDC) currently rely on artificial structuring of the material nonlinearity to satisfy phase-matching conditions. This technique, known as periodic poling, is available only in a limited number of material platforms and introduces additional fabrication steps and errors, which are detrimental to up-scaling efforts. Here, we present a device architecture that enables SPDC of a wide range of frequencies without the need for periodic poling. We present explicit designs and calculations for 4H Silicon Carbide on-insulator, in which SPDC photon generation is so far unavailable, and thin-film Lithium Niobate on-insulator, a state-of-the-art quantum photonics platform. Our design, based on mode conversion and subsequent modal phase-matched SPDC, facilitates a CMOS compatible χ(2)χ^{(2)}χ(2) platform, and simplifies photon sources by removing the requirement of periodic poling and the associated additional fabrication complexity.
Thin-film lithium niobate (TFLN) waveguides have emerged as a pivotal platform for on-chip spontaneous parametric down-conversion (SPDC), serving as a crucible for the generation of entangled photon pairs. The periodic poling of TFLN, while capable of generating high-efficiency SPDC, demands intricate fabrication processes that can be onerous in terms of scalability and manufacturability. In this work, we introduce a novel approach to the generation of entangled photon pairs via SPDC within TFLN waveguides, harnessing the principles of modal phase-matching (MPM). To address the challenge of efficiently exciting pump light typically in a higher-order mode, we have engineered a mode converter that couples two asymmetrically dimensioned waveguides. This converter adeptly transforms the fundamental mode into a higher-order mode, demonstrating a conversion loss of 1.55 dB at 785 nm with a 3 dB bandwidth exceeding 30 nm. Subsequently, we have showcased the device's capabilities by characterizing the pair generation rate (PGR), coincidences-to-accidentals ratio (CAR), and spectral profile of the entangled photon source. Our findings present a simplified and versatile method for the on-chip generation of entangled photon sources, which may pave the way for the application in the realms of quantum information processing and communication technologies.
Spontaneous parametric down-conversion (SPDC) has been utilized in generating non-classical light, such as correlated photon pairs and squeezed light, significantly influencing optical quantum information research. Advances in thinfilm lithium niobate (TFLN) technologies have brought the benefits of χ(2)nonlinearity from bulk optics to integrated photonics. Targeting fully exploitation of high Electro-optic (EO) controllability and the periodic poled lithium niobate (PPLN), enabling quasi-phase matching (QPM), offer the realization especially under the X-cut based platform [1]–[3]. Moreover, the small footprint is also key figure of merit for efficient processing of SPDC spectrum. In this work, we demonstrate SPDC photon pair generation in a small footprint size with a 0.614 nm FSR at the IR wavelength, based on X-cut PPLN racetrack resonator.
Abstract Spontaneous parametric down-conversion (SPDC), especially in non-linear waveguides, serves as an important process to generate quantum states of light with desired properties. In this work, we report on a design of a strongly dispersive, singly poled thin film lithium niobate (TFLN) waveguide geometry which acts as a convertible source of photon pairs across telecom band with tunable spectral properties. Through our simulations, we demonstrate that by using this optimized waveguide geometry, two completely different yet desirable type II phase-matched SPDC processes are enabled using a single poling period. One process generates spectrally correlated non-degenerate photon pairs with one photon at 1310nm (telecom O band) and the other at 1550nm (telecom C band). The second SPDC process results in spectrally uncorrelated photon pairs in telecom C band at 1533nm and 1567nm respectively.We attribute this versatility of TFLN waveguide to its strong dispersion properties and make a comparative study with the existing weakly dispersive waveguide platforms.We believe that such a versatile source of photon pairs will serve as an important ingredient in various quantum optical tasks which require photons at different telecom bands and desired spectral properties.
Abstract Broadband photon pairs are highly desirable for quantum metrology, quantum sensing, and quantum communication. Such sources are usually designed through type-0 phase-matching spontaneous parametric down-conversion (SPDC) that makes the photon pairs hard to separate in the frequency-degenerate case and thus limits their applications. In this paper, we design a broadband frequency-degenerate telecom-band photon pair source via the type-II SPDC in a dispersion-engineered thin-film lithium niobate waveguide, where the polarization modes of photon pairs are orthogonal and thus are easily separated deterministically. With a 5-mm-long waveguide, our design can achieve a bandwidth of 5.56 THz (44.8 nm), which is 8.6 times larger than that of the bulk lithium niobate, and the central wavelength can be flexibly adjusted. Our design is a promising approach towards high-quality integrated photon sources and may have wide applications in photonic quantum technologies.