© 2026 Aqora Quantum S.A.S.

Terms Privacy Legal Notice

Optica Quantum

Ultra-low-noise heralded single photons from counter-propagating SPDC

Rodrigo Amorim, Tim Julian Wörmann, Andrius Žukauskas, Lars Grüner-Nielsen, Valdas Pašiškevičius, Karsten K. Rottwitt

Published Jul 28, 2026• 0 citations

Abstract

Over the past decade, counter-propagating spontaneous parametric down-conversion (CPSPDC) has attracted interest due to its ability to generate narrowband and spectrally pure photon pairs without the need for strong spectral filtering. While prior work has established the spectral characteristics of CPSPDC, its performance as a heralded single-photon source (HSPS) remains limited. Here, we present a detailed HSPS characterization of a first-order quasi-phase-matched CPSPDC source based on a sub-μm periodically poled potassium titanyl phosphate crystal. We measure an internal pair-generation rate of 1.3×10 3 pairs/s/mW under CW pumping, in addition to a high coincidence-to-accidental ratio exceeding 1.4×10 5 , and furthermore obtain the lowest heralded second-order correlation value of (8.57±4.95)×10 -6 ever reported. We also show a Hong–Ou–Mandel interference experiment that yields a raw visibility of 70±1%, confirming photon indistinguishability. These results demonstrate CPSPDC as a strong candidate for generating high-quality single photons for quantum information processing and long-distance quantum communication applications.

DOI: 10.1364/opticaq.601292

Related papers

Counter-Propagating SPDC Photon Pair Generation with High Coincidence-to-Accidental Ratio

Tim Julian Wörmann, Rodrigo Amorim, Andrius Žukauskas +3 more

We demonstrate counter-propagating spontaneous parametric down-conversion in a sub-micron periodically poled Rb-doped KTP crystal, matching the highest reported coincidence-to-accidental ratio. A first heralded g H (2) measurement confirms the high-purity single-photon generation.

•Jan 1, 2026•1 citation

Heralded single-photon source utilizing highly nondegenerate, spectrally factorable spontaneous parametric downconversion

Fumihiro Kaneda, Karina Garay-Palmett, Alfred B. U’Ren +1 more

We report on the generation of an indistinguishable heralded single-photon state, using highly nondegenerate spontaneous parametric downconversion (SPDC). Spectrally factorable photon pairs can be generated by incorporating a broadband pump pulse and a group-velocity matching (GVM) condition in a periodically-poled potassium titanyl phosphate (PPKTP) crystal. The heralding photon is in the near IR, close to the peak detection efficiency of off-the-shelf Si single-photon detectors; meanwhile, the heralded photon is in the telecom L-band where fiber losses are at a minimum. We observe spectral factorability of the SPDC source and consequently high purity (90%) of the produced heralded single photons by several different techniques. Because this source can also realize a high heralding efficiency (> 90%), it would be suitable for time-multiplexing techniques, enabling a pseudo-deterministic single-photon source, a critical resource for optical quantum information and communication technology.

arXiv:1603.08451•Optics Express•May 9, 2016•113 citations

Counter-propagating spectrally uncorrelated biphotons at 1550 nm generated from periodically poled MTiOXO4 (M = K, Rb, Cs; X = P, As)

Wu‐Hao Cai, Bei Wei, Shun Wang +1 more

We theoretically investigated spectrally uncorrelated biphotons generated in a counter-propagating spontaneous parametric downconversion (CP-SPDC) from periodically poled M T i O X O 4 ( M = K , Rb, Cs; X = P , As) crystals. By numerical calculation, it was found that the five crystals from the KTP family can be used to generate heralded single photons with high spectral purity and wide tunability. Under the type-0 phase-matching condition, the purity at 1550 nm was between 0.91 and 0.92, and the purity can be maintained over 0.90 from 1500 nm to 2000 nm wavelength. Under the type-II phase-matching condition, the purity at 1550 nm was 0.96, 0.97, 0.97, 0.98, and 0.98 for PPKTP, PPRTP, PPKTA, PPRTA, and PPCTA, respectively; furthermore, the purity can be kept over 0.96 for more than 600 nm wavelength range. We also simulated the Hong–Ou–Mandel interference between independent photon sources for PPRTP crystals at 1550 nm, and interference visibility was 92% (97%) under the type-0 (type-II) phase-matching condition. This study may provide spectrally pure narrowband single-photon sources for quantum memories and quantum networks at telecom wavelengths.

arXiv:2009.13321•Journal of the Optical Society of America B•Aug 19, 2020•10 citations

Experimental verification of high spectral entanglement for pulsed waveguided spontaneous parametric down-conversion

Malte Avenhaus, Maria V. Chekhova, Leonid A. Krivitsky +2 more

We study the spectral properties of spontaneous parametric down-conversion (SPDC) in a periodically poled waveguided structure of potassium-titanyl-phosphate (KTP) crystal pumped by ultrashort pulses. Our theoretical analysis reveals a strongly entangled and asymmetric structure of the two-photon spectral amplitude for type-II SPDC. We confirm these predictions experimentally by measuring single-photon spectra, on one hand, and the dependence of Hong-Ou-Mandel interference visibility on the width of spectral filtering, on the other hand.

arXiv:0810.0998•Physical Review A•Apr 30, 2009•49 citations

Counter-propagating spontaneous parametric down-conversion source in lithium niobate on insulator

Jost Kellner, Alessandra Sabatt, Tristan Kuttner +2 more

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.

•Apr 25, 2026•0 citations

Simulating single-photon sources based on backward-wave spontaneous parametric down-conversion in a periodically poled KTP waveguide

Anatoly Shukhin, D O Akat’ev, Латыпов Ильнур Зиннурович +2 more

The properties of the backward-wave spontaneous parametric down-conversion (SPDC) in a periodically poled potassium titanyl phosphate (KTP) waveguide are studied in the context of creating narrowband heralded sources of single-photon states. The effective index of refraction and spatial profile of different waveguide modes, efficiency of different SPDC processes and purity of heralded photons are calculated numerically for a given waveguide. Compared to the usual co-propagating SPDC, spectral narrowing of the backward- wave SPDC was observed as should be expected.

Journal of Physics Conference Series•May 7, 2015•5 citations

Over the past decade, counter-propagating spontaneous parametric down-conversion (CPSPDC) has attracted interest due to its ability to generate narrowband and spectrally pure photon pairs without the need for strong spectral filtering. While prior work has established the spectral characteristics of CPSPDC, its performance as a heralded single-photon source (HSPS) remains limited. Here, we present a detailed HSPS characterization of a first-order quasi-phase-matched CPSPDC source based on a sub-μm periodically poled potassium titanyl phosphate crystal. We measure an internal pair-generation rate of 1.3×10 3 pairs/s/mW under CW pumping, in addition to a high coincidence-to-accidental ratio exceeding 1.4×10 5 , and furthermore obtain the lowest heralded second-order correlation value of (8.57±4.95)×10 -6 ever reported. We also show a Hong–Ou–Mandel interference experiment that yields a raw visibility of 70±1%, confirming photon indistinguishability. These results demonstrate CPSPDC as a strong candidate for generating high-quality single photons for quantum information processing and long-distance quantum communication applications.

We report on the generation of an indistinguishable heralded single-photon state, using highly nondegenerate spontaneous parametric downconversion (SPDC). Spectrally factorable photon pairs can be generated by incorporating a broadband pump pulse and a group-velocity matching (GVM) condition in a periodically-poled potassium titanyl phosphate (PPKTP) crystal. The heralding photon is in the near IR, close to the peak detection efficiency of off-the-shelf Si single-photon detectors; meanwhile, the heralded photon is in the telecom L-band where fiber losses are at a minimum. We observe spectral factorability of the SPDC source and consequently high purity (90%) of the produced heralded single photons by several different techniques. Because this source can also realize a high heralding efficiency (> 90%), it would be suitable for time-multiplexing techniques, enabling a pseudo-deterministic single-photon source, a critical resource for optical quantum information and communication technology.

We theoretically investigated spectrally uncorrelated biphotons generated in a counter-propagating spontaneous parametric downconversion (CP-SPDC) from periodically poled M T i O X O 4 ( M = K , Rb, Cs; X = P , As) crystals. By numerical calculation, it was found that the five crystals from the KTP family can be used to generate heralded single photons with high spectral purity and wide tunability. Under the type-0 phase-matching condition, the purity at 1550 nm was between 0.91 and 0.92, and the purity can be maintained over 0.90 from 1500 nm to 2000 nm wavelength. Under the type-II phase-matching condition, the purity at 1550 nm was 0.96, 0.97, 0.97, 0.98, and 0.98 for PPKTP, PPRTP, PPKTA, PPRTA, and PPCTA, respectively; furthermore, the purity can be kept over 0.96 for more than 600 nm wavelength range. We also simulated the Hong–Ou–Mandel interference between independent photon sources for PPRTP crystals at 1550 nm, and interference visibility was 92% (97%) under the type-0 (type-II) phase-matching condition. This study may provide spectrally pure narrowband single-photon sources for quantum memories and quantum networks at telecom wavelengths.

We study the spectral properties of spontaneous parametric down-conversion (SPDC) in a periodically poled waveguided structure of potassium-titanyl-phosphate (KTP) crystal pumped by ultrashort pulses. Our theoretical analysis reveals a strongly entangled and asymmetric structure of the two-photon spectral amplitude for type-II SPDC. We confirm these predictions experimentally by measuring single-photon spectra, on one hand, and the dependence of Hong-Ou-Mandel interference visibility on the width of spectral filtering, on the other hand.

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.

The properties of the backward-wave spontaneous parametric down-conversion (SPDC) in a periodically poled potassium titanyl phosphate (KTP) waveguide are studied in the context of creating narrowband heralded sources of single-photon states. The effective index of refraction and spatial profile of different waveguide modes, efficiency of different SPDC processes and purity of heralded photons are calculated numerically for a given waveguide. Compared to the usual co-propagating SPDC, spectral narrowing of the backward- wave SPDC was observed as should be expected.