Spatial entanglement of paired photons generated in cold atomic ensembles

TL;DR

This paper investigates how the propagation directions of photons generated in cold atomic ensembles influence their spatial quantum states and entanglement, revealing that directional information can affect the purity of other entanglement degrees of freedom.

Contribution

It demonstrates the role of photon propagation direction in shaping spatial entanglement and its impact on the overall quantum state in cold atomic ensemble photon sources.

Findings

Photon propagation direction determines the spatial quantum state.

Directional information can degrade polarization or frequency entanglement.

Spatial entanglement depends on the emission geometry.

Abstract

Cold atomic ensembles can mediate the generation of entanglement between pairs of photons. Photons with specific directions of propagation are detected, and the entanglement can reside in any of the degrees of freedom that describe the whole quantum state of the photons: polarization, spatial shape or frequency. We show that the direction of propagation of the generated photons determines the spatial quantum state of the photons and therefore, the amount of entanglement generated. When photons generated in different directions are combined, this spatial distinguishing information can degrade the quantum purity of the polarization or frequency entanglement.