Exploring photon-number-encoded high-dimensional entanglement from a sequentially excited quantum three-level system

The sequential resonant excitation of a two-level quantum system results in the emission of a state of light showing time-entanglement encoded in the photon-number basis: notions that can be extended to three-level quantum systems as discussed in a recent proposal [ Opt. Lett. 48 , 6332 ( 2023 ) 10.1364/OL.506403 ]. Here, we report the experimental implementation of a sequential two-photon resonant excitation process of a solid-state three-level system, constituted by the biexciton, exciton, and ground states of a semiconductor quantum dot. The resulting light state exhibits entanglement in time and energy, encoded in the photon-number basis, which could be used in quantum information applications, e.g., dense information encoding or quantum communication protocols. By performing energy- and time-resolved correlation experiments in combination with extensive theoretical modeling, we are able to partially retrieve the entanglement structure of the generated state and extract an upper bound for the fidelity to the entangled target state of F ≤70% before loss.