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arXiv:2602.04646

Pure narrowband photon-pair generation in a monolithic cavity

Xavier Barcons Planas, Helen M. Chrzanowski, Janik Wolters

quant-phSubmitted Feb 4, 2026• Announced Feb 5, 2026• 0 citations
v1 · Feb 4, 2026v2 · May 8, 2026
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Abstract

Photonic quantum technologies require efficient sources of pure single photons. Here we present a heralded SPDC single-photon source 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 84%, while keeping the multi-photon contamination below 3%. The cavity enhancement generates photons mainly in the central cavity mode with 96.2% spectral purity.

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arXiv:1908.00590•Optics Express•Dec 31, 2019•23 citations
Photonic quantum technologies require efficient sources of pure single photons. Here we present a heralded SPDC single-photon source 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 84%, while keeping the multi-photon contamination below 3%. The cavity enhancement generates photons mainly in the central cavity mode with 96.2% spectral purity.
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)%.
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.
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.
Cavity-enhanced spontaneous parametric down-conversion (SPDC) can be used to implement heralded single-photon sources with narrow bandwidth down to tens of megahertz, thus compatible with atom-based quantum memories. In this work, we propose and study the cavities that are either singly resonant at the frequency of only one of the generated photons or doubly resonant at the frequency of one generated photon and for the pump field. We derive analytical expressions for the generation rate and the spectral brightness as a function of the main structure parameters. Our analysis shows that by exploiting counterpropagating SPDC inside a cavity with properly designed distributed Bragg reflectors, pure narrow-band heralded single photons can be generated. In particular, when a pulsed pump illuminates a 1-mm-long singly resonant cavity, the bandwidth of the resonant and the nonresonant photons will be 120 MHz and 5.7 GHz, respectively, with heralded single-photon purity as high as 0.993. Due to its small absorption probability inside the cavity, the heralding efficiency can approach unity if the nonresonant partner is used as a heralded single photon. Finally, we also show that continuous-wave excitation of a 1-cm-long cavity resonant for the pump and copropagating fields can provide photon pairs in a narrow bandwidth of 10 MHz with spectral brightness as high as $2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $({\text{s}\phantom{\rule{4.pt}{0ex}}\text{mW}\phantom{\rule{4.pt}{0ex}}\text{MHz})}^{\ensuremath{-}1}$. This source can efficiently couple with quantum memories and can find applications in quantum information processing and communication.
We present an efficient and robust source of photons at the 87 Rb D1-line (795 nm) with a narrow bandwidth of δ = 226(1) MHz. The source is based on non-degenerate, cavity-enhanced spontaneous parametric down-conversion in a monolithic optical parametric oscillator far below threshold. The setup allows for efficient coupling to single mode fibers. A heralding efficiency of η heralded = 45(5) % is achieved, and the uncorrected number of detected photon pairs is 3.8 × 10 3 /(s mW). For pair generation rates up to 5 × 10 5 /s, the source emits heralded single photons with a normalized, heralded, second-order correlation function g c ( 2 ) < 0.01 . The source is intrinsically stable due to the monolithic configuration. Frequency drifts are on the order of δ /20 per hour without active feedback on the emission frequency. We achieved fine-tuning of the source frequency within a range of >2 GHz by applying mechanical strain.