Quantum Dot Biexciton Cascade — Entangled Photon Source

A semiconductor quantum dot pumped into its biexciton state relaxes through a two-photon radiative cascade — XX→X, then X→0 — that is one of the leading solid-state routes to on-demand single photons and polarization-entangled photon pairs. This simulator plays out that cascade in 3D: pump the dot, watch photon 1 launch, watch a polarization arrow precess through the intermediate exciton state at a rate set by the electron-hole exchange fine-structure splitting δ, then watch photon 2 launch with a polarization correlated to (or decorrelated from) the first depending on how long the dot lingered and which basis you measure in. Live statistics build up the measured correlation against the exact theoretical curve, and a post-selection timing gate shows how real experiments recover high-fidelity entanglement from an imperfect dot by keeping only the fast-decaying pairs.