Enhancing the purity of single photons in parametric down-conversion through simultaneous pump-beam and crystal-domain engineering

TL;DR

This paper demonstrates that simultaneous pump-beam and crystal-domain engineering in SPDC can significantly enhance the purity of single photons, achieving up to 99% purity without spatial filtering, which is crucial for quantum information applications.

Contribution

The study introduces a comprehensive method combining pump and crystal engineering to minimize spatial correlations in SPDC, surpassing previous approaches that focused solely on crystal domain design.

Findings

Achieved up to 99% single-photon purity without spatial filtering.

Gaussian nonlinear response reduces but does not eliminate spatial correlations.

Simultaneous pump and crystal engineering effectively minimizes correlations.

Abstract

Spontaneous parametric down-conversion (SPDC) has shown great promise in the generation of pure and indistinguishable single photons. Photon pairs produced in bulk crystals are highly correlated in terms of transverse space and frequency. These correlations limit the indistinguishability of photons and result in inefficient photon sources. Domain-engineered crystals with a Gaussian nonlinear response have been explored to minimize spectral correlations. Here, we study the impact of such domain engineering on spatial correlations of generated photons. We show that crystals with a Gaussian nonlinear response reduce the spatial correlations between photons. However, the Gaussian nonlinear response is not sufficient to fully eliminate the spatial correlations. Therefore, the development of a comprehensive method to minimize these correlations remains an open challenge. Our solution to this problem involves simultaneous engineering of the pump beam and crystal. We achieve purity of single-photon state up to 99 \% without any spatial filtering. Our findings provide valuable insights into the spatial waveform generated in structured SPDC crystals, with implications for applications such as boson Sampling.