# Pure down-conversion photons through sub-coherence length domain engineering

## TL;DR

This paper introduces novel crystal nonlinearity engineering techniques using sub-coherence-length domains to produce highly pure heralded single photons, significantly improving photon purity for short nonlinear crystals with femtosecond lasers.

## Contribution

It presents new methods combining deterministic and probabilistic domain engineering at sub-coherence lengths to enhance photon purity in parametric down-conversion sources.

## Key findings

- Sub-coherence-length domain engineering outperforms previous methods.
- Techniques achieve high spectral purity with short nonlinear crystals.
- Methods are particularly effective with femtosecond laser sources.

## Abstract

Photonic quantum technology relies on efficient sources of coherent single photons, the ideal carriers of quantum information. Heralded single photons from parametric down-conversion can approximate on-demand single photons to a desired degree, with high spectral purities achieved through group-velocity matching and tailored crystal nonlinearities. Here we propose crystal nonlinearity engineering techniques with sub-coherence-length domains. We first introduce a combination of two existing methods: a deterministic approach with coherence-length domains and probabilistic domain-width annealing. We then show how the same deterministic domain-flip approach can be implemented with sub-coherence length domains. Both of these complementary techniques create highly pure photons, outperforming previous methods, in particular for short nonlinear crystals matched to femtosecond lasers.