Coherent Transient Absorption Features of Hot and Cold Excitons in a Single Quantum Dot

In recent years, progress in understanding nanoemitters of nonclassical light enabled inspiring new directions in quantum science and technology. While studies of photoluminescence (PL) provide important insights, they are typically limited to the exciton ground state and to a nanosecond temporal resolution. Switching to a transient-transmission approach provides femtosecond resolution and access to hot excitons. However, this task poses formidable challenges: ultrafast pump-probe spectroscopy must be performed on an individual nanoemitter with a maximum signal of a single photon per pulse among a strong background of transmitted probe light [1], [2]. Here, we observe quantum beating between quasi-degenerate states within a single CdSe/ZnSe quantum dot directly in the time domain. Surprisingly, we find a direct path to long-lived excited superpositions far above the exciton ground state, showcasing a pronounced phonon bottleneck in the decay of distinct hot states.