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

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.
In our recent work [1], 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.
We experimentally characterise our method through a high-precision measurement of multi-photon interference between two heralded photons generated by two independent sources.
Finally, we show that our new crystal-engineering algorithm allows us to investigate more exotic cases such as complex-chirped nonlinearity profiles or antisymmetric PDC joint spectra.
[1] F. Graffitti, D. Kundys, D. T. Reid, A. M. Branczyk, and A. Fedrizzi, Quantum Science and Technology 2, 035001 (2017).