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

Suppressing Parasitic Nonlinear Processes in Frequency-Degenerate Photon Pairs Emission in Layer-Poled TFLN Waveguides

Olivia Hefti, Enrico Melani, Marco Clementi, Jean-Étienne Tremblay, Andrea Volpini, Yeşim Koyaz, Homa Zarebidaki, Iván Prieto, Olivier Dubochet, Charles Caër, Daniele Bajoni, Hamed Sattari, Camille‐Sophie Brès, Mattéo Galli, Davide Grassani

Published Jun 23, 2025• 0 citations
Publisher

Abstract

Suppression of noise photons from unwanted nonlinear processes is a major hurdle in generating frequency-degenerate correlated photon pairs for quantum information, e.g. continuous-variable quantum computing [1]. On-chip, telecom-pumped and frequency-degenerate photon pairs have been produced either using second harmonic generation (SHG) followed by spontaneous parametric down conversion (SPDC) in thin film lithium niobate (TFLN) waveguides [2], or by dual-pump spontaneous four-wave mixing (DP-SFWM) in microresonators [3], [4]. However, the former needs careful filtering of the SHG pump, while the latter suffers from single-pump (SP-) SFWM contributions, which decrease the purity of the generated quantum states. SP-SFWM can be suppressed by linearly coupling resonators at the expense of reduced bandwidth [3] or efficiency [4].

DOI
10.1109/cleo/europe-eqec65582.2025.11111320

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Suppression of noise photons from unwanted nonlinear processes is a major hurdle in generating frequency-degenerate correlated photon pairs for quantum information, e.g. continuous-variable quantum computing [1]. On-chip, telecom-pumped and frequency-degenerate photon pairs have been produced either using second harmonic generation (SHG) followed by spontaneous parametric down conversion (SPDC) in thin film lithium niobate (TFLN) waveguides [2], or by dual-pump spontaneous four-wave mixing (DP-SFWM) in microresonators [3], [4]. However, the former needs careful filtering of the SHG pump, while the latter suffers from single-pump (SP-) SFWM contributions, which decrease the purity of the generated quantum states. SP-SFWM can be suppressed by linearly coupling resonators at the expense of reduced bandwidth [3] or efficiency [4].
Frequency-degenerate photon pairs generated using nonlinear photonic integrated devices are a crucial resource for scalable quantum information processing and metrology. However, their realization based on dual-pump spontaneous four-wave mixing is hindered by unwanted parametric processes occurring within the same phase matching band, which degrade the signal-to-noise ratio and reduce the purity of the associated quantum states. Here, we propose a dual-pump scheme to produce frequency-degenerate photon pairs, based on cascaded sum-frequency generation and spontaneous parametric down-conversion occurring within a single waveguide, while strongly suppressing parasitic photon pair generation from single-pump processes. This approach significantly simplifies the design compared to microresonator-based methods and enables both pumping and collection of photon pairs entirely in the telecom band. We experimentally validate the concept in a layer-poled thin film lithium niobate waveguide, achieving frequency-degenerate photon pair generation with a brightness of (1.0±0.3)×10 5 Hz nm −1 mW −2 and a 40 dB suppression of unwanted single-pump processes.
Frequency-degenerate photon pairs generated using nonlinear photonic integrated devices are a crucial resource for scalable quantum information processing and metrology. However, their realization is hindered by unwanted parametric processes occurring within the same phase matching band, which degrade the signal-to-noise ratio and reduce the purity of the associated quantum states. Here, we propose a dual-pump scheme to produce frequency-degenerate photon pairs, based on cascaded sum-frequency generation and spontaneous parametric down-conversion occurring within a single waveguide, while strongly suppressing parasitic photon pair generation from single-pump processes. This approach significantly simplifies the design compared to microresonator-based methods and enables both pumping and collection of photon pairs entirely in the telecom band. We experimentally validate the concept in a layer-poled thin film lithium niobate waveguide, achieving frequency-degenerate photon pair generation with a brightness of \SI{1.0(3)e5}{\hertz \per \nm \per \square \milli \watt } and a 40 dB suppression of unwanted single-pump processes.
Frequency-degenerate photon pairs are a key resource for quantum information processing. Yet, their generation via nonlinear parametric processes is often accompanied by parasitic interactions that hinder state purity. We present a novel approach based on dual-pump, cascaded sum-frequency generation (SFG) and spontaneous parametric down-conversion (SPDC) within a single layer-poled thin-film lithium niobate waveguide. By exploiting modal phase matching, our scheme effectively suppresses spurious photons originating from single-pump processes. This compact, single-pass architecture operates entirely in the telecom band and eliminates the need for complex filtering or microresonator structures. Experimentally, we demonstrate degenerate photon-pair generation with a brightness of (1.0 ± 0.3) × 105 Hz nm−1 mW−2 and more than 40 dB suppression of parasitic photon-pair emission. Our results establish a scalable, low-noise platform for high-purity quantum light sources and open a pathway toward on-chip squeezed light generation via cascaded processes.
We generate quantum-correlated photon pairs using cascaded χ ( 2 ) : χ ( 2 ) traveling-wave interactions for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC) in a single periodically-poled thin-film lithium-niobate (TFLN) waveguide. When pulse-pumped at 50 MHz, a 4-mm-long poled region with nearly 300%/Wcm2 SHG peak efficiency yields a generated photon-pair probability of 7±0.2 × 10−4 with corresponding coincidence-to-accidental ratio (CAR) of 13.6±0.7. The CAR is found to be limited by Stokes/anti-Stokes Raman-scattering noise generated primarily in the waveguide. A Raman peak of photon counts at 250 cm−1 Stokes shift from the fundamental-pump wavenumber suggests most of the noise that limits the CAR originates within the lithium niobate material of the waveguide.
Pairs of entangled photons are crucial for photonic quantum technologies. The demand for integrability and multi-functionality suggests 'flat' platforms - ultrathin layers and metasurfaces - as sources of photon pairs. With the success in demonstrating spontaneous parametric down-conversion (SPDC) from such sources, an alternative process to generate photon pairs, spontaneous four-wave mixing (SFWM), also starts to attract interest. In materials with nonzero second-order nonlinear susceptibility χ(2)χ^{(2)}χ(2), SFWM can generate photon pairs both directly, through the third-order nonlinear susceptibility χ(3)χ^{(3)}χ(3), and in a cascaded way, through second harmonic generation (SHG) followed by SPDC. Usually, the cascaded process is more efficient. Here, we show that in a thin layer, direct SFWM dominates, because the wavevector mismatch for SFWM is much smaller than for SHG or SPDC. To demonstrate it, we implement the photon pair generation via SFWM in a second-order nonlinear material - a thin layer of lithium niobate (LN). The existence of both second- and third-order nonlinear processes offers broader opportunities for quantum state engineering.
Pairs of entangled photons are crucial for photonic quantum technologies. The demand for integrability and multi-functionality suggests “flat” platforms—ultrathin layers and metasurfaces—as sources of photon pairs. With the success in demonstrating spontaneous parametric down-conversion (SPDC) from such sources, an alternative process to generate photon pairs, spontaneous four-wave mixing (SFWM), also attracts interest. In materials with nonzero second-order nonlinear susceptibility χ (2) , SFWM can generate photon pairs both directly, through the third-order nonlinear susceptibility χ (3) , and in a cascaded way, through second harmonic generation (SHG) followed by SPDC. Usually, the cascaded process is more efficient. Here, we show that in a thin layer, direct SFWM is likely to dominate, because the wavevector mismatch for SFWM is much smaller than for SHG or SPDC. To demonstrate this, we implement photon pair generation via SFWM in a second-order nonlinear material—a thin layer of lithium niobate (LN). The existence of both second- and third-order nonlinear processes offers broader opportunities for quantum state engineering.