# The effects of filtering on the purity of heralded single photons from parametric sources

## TL;DR

This paper analyzes how spectral filtering affects the purity and success rate of heralded single photons from parametric sources, revealing a trade-off that can be optimized for quantum applications.

## Contribution

It provides an analytical characterization of the purity-rate trade-off for spectral filtering in parametric single-photon sources.

## Key findings

- Up to 20% heralding success with 90% photon purity.
- Spectral filtering reduces spectral correlations, improving purity.
- Trade-offs between heralding rate and photon purity are quantitatively characterized.

## Abstract

Optical quantum protocols are reliant on the production of highly pure single photons. However, most parametric heralded single photon sources possess spectral correlations, limiting the purity of the produced photon. The most common method for restoring the purity is with narrowband spectral filtering, which comes at the cost of reducing the rate of heralding detection. Here, we characterize this trade-off analytically for a large class of parametric sources. It is possible to achieve up to 20% heralding success probabilities with 90% heralded photon purities.