Single-photon superradiance in individual caesium lead halide quantum dots

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The brightness of an emitter is ultimately described by Fermi’s golden rule, with a radiative rate proportional to its oscillator strength times the local density of photonic states (LDOS). Since the oscillator strength is an intrinsic material property, the quest for ever brighter emission has relied on LDOS engineering, via dielectric or plasmonic resonators. In contrast, a much less explored avenue is to boost the oscillator strength, and hence the emission rate, via a collective behavior termed superradiance. Recently, it had been proposed that the latter can be realized via the giant oscillator-strength transitions of a weakly confined exciton in a quantum well when its coherent motion extends over many unit cells. Here, we demonstrate single-photon superradiance in perovskite quantum dots (QDs) with a sub-100 ps radiative decay time, almost as short as the reported exciton coherence time. The characteristic dependence of radiative rates on QD size, composition, and temperature suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations. The results aid the development of ultrabright, coherent quantum-light sources and attest that quantum effects, e.g., single-photon emission, persist in nanoparticles ten times larger than the exciton Bohr radius.