Wavevector-resolved polarization entanglement from radiative cascades

TL;DR

This paper reveals that polarization entanglement in radiative cascades from quantum dots depends on emission wavevector, challenging previous assumptions and guiding the design of better entangled photon sources for quantum tech.

Contribution

It demonstrates the wavevector dependence of polarization entanglement in radiative cascades and provides a theoretical model for optimizing microcavity designs.

Findings

Entanglement varies with emission angle, can vanish at large angles.

Wavevector-entanglement relationship is experimentally confirmed.

Model offers quantitative design guidelines for microcavities.

Abstract

The generation of entangled photons from radiative cascades has enabled milestone experiments in quantum information science with several applications in photonic quantum technologies. Significant efforts are being devoted to pushing the performances of near-deterministic entangled-photon sources based on single quantum emitters often embedded in photonic cavities, so to boost the flux of photon pairs. The general postulate is that the emitter generates photons in a nearly maximally entangled state of polarization, ready for application purposes. Here, we demonstrate that this assumption is unjustified. We show that in radiative cascades there exists an interplay between photon polarization and emission wavevector, strongly affecting quantum correlations when emitters are embedded in micro-cavities. We discuss how the polarization entanglement of photon pairs from a biexciton-exciton cascade in quantum dots strongly depends on their propagation wavevector, and it can even vanish for large emission angles. Our experimental results, backed by theoretical modelling, yield a brand-new understanding of cascaded emission for various quantum emitters. In addition, our model provides quantitative guidelines for designing optical microcavities that retain both a high degree of entanglement and collection efficiency, moving the community one step further towards an ideal source of entangled photons for quantum technologies.