  Quantum particles sometimes cooperate to develop a macroscopically ordered
state with extraordinary properties. Superconductivity and Bose-Einstein
condensation are examples of such cooperative phenomena where macroscopic order
appears spontaneously. Here, we demonstrate that such an ordered state can also
be obtained in an optically excited semiconductor quantum well in a high
magnetic field. When we create a dense electron-hole plasma with an intense
laser pulse, after a certain delay, an ultrashort burst of coherent radiation
emerges. We interpret this striking phenomenon as a manifestation of
superfluorescence (SF), in which a macroscopic polarization spontaneously
builds up from an initially incoherent ensemble of excited quantum oscillators
and then decays abruptly producing giant pulses of coherent radiation. SF has
been observed in atomic gases, but the present work represents the first
observation of SF in a solid-state setting. While there is an analogy between
the recombination of electron-hole pairs and radiative transitions in atoms,
there is no a priori reason for SF in semiconductors to be similar to atomic
SF. This is a complex many-body system with a variety of ultrafast
interactions, where the decoherence rates are at least 1,000 times faster than
the radiative decay rate, an unusual situation totally unexplored in previous
atomic SF studies. We show, nonetheless, that collective many-body coupling via
a common radiation field does develop under certain conditions and leads to SF
bursts. The solid-state realization of SF resulted in an unprecedented degree
of controllability in the generation of SF, opening up opportunities for both
fundamental many-body studies and device applications. We demonstrate that the
intensity and delay time of SF bursts are fully tunable through an external
magnetic field, temperature, and pump laser power.
