Two-Photon Excitation Sets Limit to Entangled Photon Pair Generation from Quantum Emitters

TL;DR

This paper reveals that two-photon excitation in quantum emitters inherently limits the maximum entanglement of generated photon pairs due to laser-induced state splitting, even without excitonic fine-structure splitting.

Contribution

It demonstrates that finite-duration excitation pulses cause a dynamic splitting of exciton states, restricting entanglement fidelity in quantum dot photon sources.

Findings

Entanglement cannot reach unity with finite-duration pulses.

Laser pulses induce a dynamic splitting of exciton states.

Excitation process limits entangled photon quality.

Abstract

Entangled photon pairs are key to many novel applications in quantum technologies. Semiconductor quantum dots can be used as sources of on-demand, highly entangled photons. The fidelity to a fixed maximally entangled state is limited by the excitonic fine-structure splitting. This work demonstrates that, even if this splitting is absent, the degree of entanglement cannot reach unity when the excitation pulse in a two-photon resonance scheme has a finite duration. The degradation of the entanglement has its origin in a dynamically induced splitting of the exciton states caused by the laser pulse itself. Hence, in the setting explored here, the excitation process limits the achievable concurrence for entangled photons generated in an optically excited four-level quantum emitter.