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)%.
@article{arxiv.2602.04646,
title = {Pure narrowband photon-pair generation in a monolithic cavity},
author = {Xavier Barcons Planas and Helen M. Chrzanowski and Janik Wolters},
journal= {arXiv preprint arXiv:2602.04646},
year = {2026}
}