  Since Dicke's seminal paper on coherence in spontaneous radiation by atomic
ensembles, superradiance has been extensively studied. Subradiance, on the
contrary, has remained elusive, mainly because subradiant states are weakly
coupled to the environment and are very sensitive to nonradiative decoherence
processes.Here we report the experimental observation of subradiance in an
extended and dilute cold-atom sample containing a large number of particles. We
use a far detuned laser to avoid multiple scattering and observe the temporal
decay after a sudden switch-off of the laser beam. After the fast decay of most
of the fluorescence, we detect a very slow decay, with time constants as long
as 100 times the natural lifetime of the excited state of individual atoms.
This subradiant time constant scales linearly with the cooperativity parameter,
corresponding to the on-resonance optical depth of the sample, and is
independent of the laser detuning, as expected from a coupled-dipole model.
