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An ensemble of emitters can behave differently from its individual constituents when it interacts coherently via common vacuum light modes. One example of a many-body collective coupling is so-called superfluorescent coupling, where the excited emitters are initially fully uncorrelated and coherence is established through spontaneously triggered correlations from quantum fluctuations. Subsequently, the coupled emitters emit a strong superfluorescent pulse. Since this phenomenon requires low inhomogeneity and a fine balance of interactions between the resonant emitters and their decoupling from the environment, superfluorescence has only been observed in a limited number of systems, such as certain atomic and molecular gases and a few solid-state systems.
Here, we investigate densely packed arrays of fully inorganic cesium lead halide perovskite quantum dots[1], known as superlattices. These quantum dots obtain exceptional optical properties such as an lowest bright triplet state with an ultrafast radiative decay that is 1000x faster compared to other conventional nanocrystals at cryogenic temperatures[2]. The resulting high oscillator strength and a long exciton dephasing time[3] are key ingredients for strong light-matter interactions. In a solvent-drying-induced assembly process, perovskite quantum dots form densely packed cuboidal superlattices that show key signatures of superfluorescence[4]. We observe a more than twenty-fold accelerated radiative decay with dynamically red-shifted emission, extension of the first-order coherence time by more than a factor of four, photon bunching and an intensity-dependent time delay after which the photon burst is emitted. Also, at high excitation density, the superfluorescent decay exhibits a Burnham-Chiao ringing behavior, reflecting the coherent Rabi-type interaction.
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