Quantum beats of exciton-polarons in CsPbI₃ perovskite nanocrystals

The optical response of semiconductors is governed by coupled electronic and vibrational excitations. In lead-halide perovskite nanocrystals, strong exciton-phonon interaction forms a ladder of exciton-polaron states accessible by femtosecond laser pulses. We demonstrate a fully coherent regime of exciton-polaron dynamics with long optical coherence times (T₂ ≈ 300 ps) in CsPbI₃ nanocrystals embedded in glass. Using transient two-pulse photon echo at a temperature of 2 K, we observe quantum beats between exciton-polaron states, with decay determined by optical phonon lifetimes of 5-15 ps. Within a four-level model, we directly quantify the exciton-phonon coupling strength through Huang-Rhys factors of 0.05 - 0.12 and 0.02 - 0.04 for low-energy optical phonons with energies of 3.2 and 5.1 meV, respectively. The pronounced size dependence of both coupling strengths and phonon lifetimes offers a route to tune the optical transitions between exciton-polaron states and tailor the coherent optical dynamics in perovskite semiconductors for solid-state quantum technologies.