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

Characterization of a Monolithic Cavity for High-Purity Single-Photon Generation via Pulsed SPDC

Xavier Barcons Planas, Helen M. Chrzanowski, Janik Wolters

Published Jun 23, 2025• 0 citations

Abstract

Photonic quantum technologies rely on two-photon interference, which is fundamentally constrained by photon purity. Consequently, the efficient generation of highly pure single photons is essential for scalability. Heralded single photons from spontaneous parametric down-conversion (SPDC) photon-pair sources are a viable option; however, multimode emission reduces both their purity and heralding efficiency. Techniques such as waveguide geometries, group velocity matching, and cavity resonators improve single-mode behaviour. A monolithic cavity design [1] has been considered for further stability, tunability and robustness. In this work, we present a photon-pair source using pulsed SPDC in a monolithic non-linear crystal cavity, where cavity confinement ensures well-defined spatial and spectral modes.

DOI: 10.1109/cleo/europe-eqec65582.2025.11111673

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Photonic quantum technologies rely on two-photon interference, which is fundamentally constrained by photon purity. Consequently, the efficient generation of highly pure single photons is essential for scalability. Heralded single photons from spontaneous parametric down-conversion (SPDC) photon-pair sources are a viable option; however, multimode emission reduces both their purity and heralding efficiency. Techniques such as waveguide geometries, group velocity matching, and cavity resonators improve single-mode behaviour. A monolithic cavity design [1] has been considered for further stability, tunability and robustness. In this work, we present a photon-pair source using pulsed SPDC in a monolithic non-linear crystal cavity, where cavity confinement ensures well-defined spatial and spectral modes.

Photonic quantum technologies require efficient sources of pure single photons. We present a heralded single-photon source based on spontaneous parametric down-conversion in a monolithic cavity optimized for high spectral and spatial purity. The source heralds single photons at a wavelength of 1540 nm and a spectral bandwidth of 168 MHz, with a maximum heralding efficiency of 70% including all transmission and detection losses, while keeping the multi-photon contamination below 3%. The cavity enhancement predominantly generates photons into the central cavity mode, with a theoretical upper bound on the spectral purity of 79.4% arising from nonzero overlap with adjacent cavity modes. Spectral isolation of the central cavity mode with an etalon yields an increased measured spectral purity of (96.2

\pm

2.7)%.

Photonic quantum technologies require efficient sources of pure single photons. We present a heralded single-photon source based on spontaneous parametric down-conversion in a monolithic cavity optimized for high spectral and spatial purity. The source heralds single photons at a wavelength of 1540 nm and a spectral bandwidth of 168 MHz, with a maximum heralding efficiency of 70% including all transmission and detection losses, while keeping the multi-photon contamination below 3%. The cavity enhancement predominantly generates photons into the central cavity mode, with a theoretical upper bound on the spectral purity of 79.4% arising from nonzero overlap with adjacent cavity modes. Spectral isolation of the central cavity mode with an etalon yields an increased measured spectral purity of (96.2±2.7)%.

Efficiently generating photon pairs with high heralding efficiency and high single photon purity that are bandwidth matched to quantum emitters, quantum memories, and other matter-based qubits is critical for quantum networking applications. However, nonlinear optics-based sources require substantial spectral engineering to overcome the orders of magnitude bandwidth mismatch between those sources and qubit systems. A popular solution is cavity-enhanced spontaneous parametric down conversion (SPDC) where the cavity sets the photon bandwidth and simultaneously enhances the spectral brightness of the SPDC. Bulk, free-space configurations are generally required to achieve the MHz-scale bandwidths required to interface with most qubit systems. Replicating these in scalable integrated photonic architectures is an ongoing challenge due to the much higher propagation losses that limit the size and linewidth of chip-based resonators. We show here how an intra-cavity slow light medium, acting as an ultra-narrow filter, would enable narrowband photon pair generation in broadband cavities with high single photon purity and without compromising the heralding efficiency. We show that such metrics can be readily realized in erbium doped thin-film lithium niobate microrings using realistic design parameters.