  An ensemble of emitters can behave significantly different from its
individual constituents when interacting coherently via a common light field.
After excitation, collective coupling gives rise to an intriguing many-body
quantum phenomenon, resulting in short, intense bursts of light: so-called
superfluorescence. Because it requires a fine balance of interaction between
the emitters and their decoupling from the environment, together with close
identity of the individual emitters, superfluorescence has thus far been
observed only in a limited number of systems, such as atomic and molecular
gases and semiconductor crystals, and could not be harnessed for applications.
For colloidal nanocrystals, however, which are of increasing relevance in a
number of opto-electronic applications, the generation of superfluorescent
light was precluded by inhomogeneous emission broadening, low oscillator
strength, and fast exciton dephasing. Using caesium lead halide (CsPbX3, X =
Cl, Br) perovskite nanocrystals that are self-organized into highly ordered
three-dimensional superlattices allows us to observe key signatures of
superfluorescence: red-shifted emission with more than ten-fold accelerated
radiative decay, extension of the first-order coherence time by more than a
factor of four, photon bunching, and delayed emission pulses with Burnham-Chiao
ringing behaviour at high excitation density. These mesoscopically extended
coherent states can be employed to boost opto-electronic device performances
and enable entangled multi-photon quantum light sources.
