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arXiv:2501.14214 · Advanced Physics Research

Optimized Spectral Purity of Heralded Single Photons at the Telecom O‐Band

Wu‐Hao Cai, So-Young Baek, Rui‐Bo Jin, Fumihiro Kaneda

Published Nov 25, 2025• 1 citation
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Abstract

ABSTRACT We report on optimizing the spectral purity of heralded single photons in the telecom O‐band, where single photons can be propagated with low loss and low dispersion in a standard telecom optical fiber. We numerically searched for various group‐velocity‐matching conditions and corresponding optimal poling structures of a potassium titanyl phosphate crystal for spontaneous parametric downconversion. Our poling optimization results using phase‐matching coherence‐length and sub‐coherence‐length modulation schemes show % spectral purity with pump wavelengths ranging from 603.8 to 887.3 nm. Some optimized configurations are feasible with off‐the‐shelf lasers and single‐photon detectors. Moreover, by investigating noise photon spectra for different poling optimization methods, we show that, in practice, appropriate, gentle spectral filtering helps achieve high purity. This study will pave the way for developing practical quantum sources for quantum information applications at the telecom O‐band.

DOI
10.1002/apxr.202500167

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ABSTRACT We report on optimizing the spectral purity of heralded single photons in the telecom O‐band, where single photons can be propagated with low loss and low dispersion in a standard telecom optical fiber. We numerically searched for various group‐velocity‐matching conditions and corresponding optimal poling structures of a potassium titanyl phosphate crystal for spontaneous parametric downconversion. Our poling optimization results using phase‐matching coherence‐length and sub‐coherence‐length modulation schemes show % spectral purity with pump wavelengths ranging from 603.8 to 887.3 nm. Some optimized configurations are feasible with off‐the‐shelf lasers and single‐photon detectors. Moreover, by investigating noise photon spectra for different poling optimization methods, we show that, in practice, appropriate, gentle spectral filtering helps achieve high purity. This study will pave the way for developing practical quantum sources for quantum information applications at the telecom O‐band.
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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 report on the generation of indistinguishable photon pairs at telecom wavelengths based on a type-II parametric down conversion process in a periodically poled potassium titanyl phosphate (PPKTP) crystal. The phase matching, pump laser characteristics and coupling geometry are optimised to obtain spectrally uncorrelated photons with high coupling efficiencies. Four photons are generated by a counter-propagating pump in the same crystal and anlysed via two photon interference experiments between photons from each pair source as well as joint spectral and g((2)) measurements. We obtain a spectral purity of 0.91 and coupling efficiencies around 90% for all four photons without any filtering. These pure indistinguishable photon sources at telecom wavelengths are perfectly adapted for quantum network demonstrations and other multi-photon protocols.
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