Fundamental limit on the heralded single photons' spectral brightness

TL;DR

This paper establishes a fundamental limit on the spectral brightness of heralded single photons, introduces a quality factor to evaluate source performance, and reports record-high brightness and quality factor using hot atomic vapor sources.

Contribution

The study introduces the concept of a quality factor to quantify how close HSP sources are to the ideal noise-free limit and provides a universal limit on spectral brightness.

Findings

Achieved record spectral brightness of (7.0±0.3)×10^5 pairs/s/MHz.

Defined the quality factor to evaluate HSP source performance.

Provided a universal limit on the spectral brightness of heralded single photons.

Abstract

The heralded single photons' (HSPs) spectral brightness (SB) is defined as the generation rate per linewidth. As the generation rate of HSPs gets larger or the photons' linewidth becomes narrower, both of which are desirable in quantum information processing using HSPs, does the SB have a limit? We systematically studied the SB and the cross-correlation function, or equivalently, the signal-to-background ratio. The results in this study provide an answer applicable to all types of HSP sources. The answer relies on a newly defined quantity, the quality factor, which reveals how a HSP source approaches the ideal noise-free one. Furthermore, employing the HSP source based on hot atomic vapor, we achieved an SB of (7.0$\pm$0.3)$\times10^5$ pairs/s/MHz and a quality factor of 0.68$\pm$0.02 under the single-photon criterion. Both values are the highest records to date among all kinds of HSP sources.