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arXiv:2608.03950

Joint spectral characterization of SPDC photon pairs near 2 $μ$m in (Al)GaAs-on-insulator waveguides

Alexandre Z. Leger, Emil Z. Ulsig, Samuel E. Fontaine, Dileep V. Reddy, Eric J. Stanton, Lynden K. Shalm, Richard P. Mirin, Martin J. Stevens, J. E. Sipe, Nicolas Volet, Deny R. Hamel

physics.opticsquant-phSubmitted Aug 4, 2026• Announced Aug 5, 2026• 0 citations
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Abstract

Integrated photon-pair sources are a core component of chip-based quantum computing, communication, and metrology. Although such sources have been demonstrated at conventional telecom wavelengths, the 2 $μ$m band remains comparatively less explored, despite offering advantages for free-space quantum communication, low-loss transmission in emerging fiber networks, and integration with silicon photonic platforms. In this paper, we demonstrate spontaneous parametric down-conversion (SPDC) in straight GaAs- and AlGaAs-on-insulator waveguides. This platform offers strong second-order nonlinearity and geometry-tunable dispersion, which are advantageous for efficient on-chip pair generation. Measurements of the joint spectral intensity and heralded second-order correlation function show broadband emission around 2 $μ$m with strong spectral anti-correlations. To our knowledge, this is the first direct joint-spectral characterization of an integrated SPDC source in this wavelength regime.

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14 pages, 8 figures, 2 tables

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Joint spectral characterization of SPDC photon pairs near 2 μμμm in (Al)GaAs-on-insulator waveguides
Integrated photon-pair sources are a core component of chip-based quantum computing, communication, and metrology. Although such sources have been demonstrated at conventional telecom wavelengths, the 2 μμμm band remains comparatively less explored, despite offering advantages for free-space quantum communication, low-loss transmission in emerging fiber networks, and integration with silicon photonic platforms. In this paper, we demonstrate spontaneous parametric down-conversion (SPDC) in straight GaAs- and AlGaAs-on-insulator waveguides. This platform offers strong second-order nonlinearity and geometry-tunable dispersion, which are advantageous for efficient on-chip pair generation. Measurements of the joint spectral intensity and heralded second-order correlation function show broadband emission around 2 μμμm with strong spectral anti-correlations. To our knowledge, this is the first direct joint-spectral characterization of an integrated SPDC source in this wavelength regime.
Quantum photonic technologies rely on the ability to generate, manipulate, and interfere indistinguishable single photons on a scalable platform. Among the various approaches, spontaneous parametric down-conversion (SPDC) remains one of the most widely used methods for generating entangled photon pairs and pure heralded single photons. However, most integrated SPDC sources relying on co-propagating geometries have a limited purity of heralded photons or require lossy filtering. Type-2 SPDC processes can produce pure separable photons but typically suffer from lower efficiency and added complexity due to polarization management. Here we show the first integrated counter-propagating photon-pair source on lithium niobate on insulator, where signal and idler photons are generated in opposite directions. The counter-propagating geometry leads to spectrally uncorrelated photon pairs without spectral filtering. The joint spectral intensity measurements and unheralded g (2) correlations yield purities of (92 ± 3)%. Interference between two independent sources achieves heralded visibilities of (71 ± 3)%, confirming the scalability of the platform. These results establish a new route towards integrated, high-purity, and tunable photon sources. The demonstrated counter-propagating geometry offers a scalable solution for quantum photonic networks.
Quantum photonic technologies rely on the ability to generate, manipulate, and interfere indistinguishable single photons on a scalable platform. Among the various approaches, spontaneous parametric down-conversion (SPDC) remains one of the most widely used methods for generating entangled photon pairs and pure heralded single photons. However, most integrated SPDC sources relying on co-propagating geometries have a limited purity of heralded photons or require lossy filtering. Type-2 SPDC processes can produce pure separable photons but typically suffer from lower efficiency and added complexity due to polarization management. Here we show the first integrated counter-propagating photon-pair source on lithium niobate on insulator, where signal and idler photons are generated in opposite directions. The counter-propagating geometry leads to spectrally uncorrelated photon pairs without spectral filtering. The joint spectral intensity measurements and unheralded g(2) correlations yield purities of (92 ± 3)degrees lc. Interference between two independent sources achieves heralded visibilities of (71 ± 3)degrees lc, confirming the scalability of the platform. These results establish anew route towards integrated, high-purity, and tunable photon sources. The demonstrated counter-propagating geometry offers a scalable solution for quantum photonic networks.
Widespread commercial adoption of telecom-band quantum-key-distribution (QKD) will require fully integrated, room-temperature transmitters. Implementing highly efficient spontaneous parametric down-conversion (SPDC) on a platform that offers co-integration of the pump laser has been an outstanding challenge. Here, using such a platform based on AlGaAs-on-insulator waveguides, we report telecom-band SPDC (and second harmonic generation) with exceedingly large efficiencies of 26 GHz generated pairs/mW over a 7 THz bandwidth, which would saturate the usable photon-flux for a 70-channel wavelength-multiplexed QKD-system at merely 1.6 mW of pump laser power.
The ability to determine the joint spectral properties of photon pairs produced by the processes of spontaneous parametric downconversion (SPDC) and spontaneous four-wave mixing (SFWM) is crucial for guaranteeing the usability of heralded single photons and polarization-entangled pairs for multi-photon protocols. In this paper, we compare six different techniques that yield either a characterization of the joint spectral intensity or of the closely related purity of heralded single photons. These six techniques include: (i) scanning monochromator measurements, (ii) a variant of Fourier transform spectroscopy designed to extract the desired information exploiting a resource-optimized technique, (iii) dispersive fibre spectroscopy, (iv) stimulated-emission-based measurement, (v) measurement of the second-order correlation function g(2) for one of the two photons, and (vi) two-source Hong–Ou–Mandel interferometry. We discuss the relative performance of these techniques for the specific cases of a SPDC source designed to be factorable and SFWM sources of varying purity, and compare the techniques’ relative advantages and disadvantages.
Spontaneous parametric down-conversion (SPDC) is one of the most important sources for entangled or correlated photon pairs, which are the foundation of the emerging fields of quantum-optical imaging and spectroscopy. Using periodically poled materials as an SPDC-source allows broad emission spectra in widely tunable wavelength ranges in an efficient and walk-off-free process. For applications that employ nonlinear interference setups, maximum interferometric contrast is only achieved when signal and idler modes are overlapped precisely in the second crystal. Thus, accurate knowledge of the spectral-angular characteristic of SPDC is crucial for designing such devices.