Quantum Drop Formation in Perovskite Quantum Dots: From Polaron-Induced Coherent States to Time-Reversed Superfluorescence and Long-Lived Electronic Coherence

We present a unified view of cooperative quantum dynamics in lead–halide perovskite quantum dots revealed by femtosecond coherent multidimensional spectroscopy. On ultrafast 50–150 fs timescales, Landau polaron fields reorganize the excitonic spectrum into a confined manifold of quantum-coherent states—a "quantum drop"—whose splittings exceed single-particle limits, scale anomalously with size, and remain robust at 300 K. From this emergent state follow two remarkable consequences. First, we observe the time reversal of superfluorescence (TRSF), in which ensembles display a delayed cooperative absorption process gated by polaron-induced lattice fields that synchronize unit-cell dipoles into a coherently phased array; the fidelity of this time-reversed process approaches 99% even under ambient conditions, establishing lattice fields as mediators of cooperative coherence. Second, we detect long-lived electronic coherence between exciton states that persists nearly three times longer than the single-particle dephasing time, arising from nearly perfectly correlated lattice fluctuations that act to stabilize superpositions. Together, these results demonstrate that perovskite quantum dots naturally harness lattice–exciton interactions to generate, protect, and manipulate collective quantum coherence at room temperature, establishing the concept of quantum drop formation as a gateway to cooperative quantum optics, engineered photonic states, and new paradigms such as quantum batteries.