  Superradiance has been extensively studied in the 1970s and 1980s in the
regime of superfluores-cence, where a large number of atoms are initially
excited. Cooperative scattering in the linear-optics regime, or "single-photon
superradiance" , has been investigated much more recently, and superra-diant
decay has also been predicted, even for a spherical sample of large extent and
low density, where the distance between atoms is much larger than the
wavelength. Here, we demonstrate this effect experimentally by directly
measuring the decay rate of the off-axis fluorescence of a large and dilute
cloud of cold rubidium atoms after the sudden switch-off of a low-intensity
laser driving the atomic transition. We show that, at large detuning, the decay
rate increases with the on-resonance optical depth. In contrast to forward
scattering, the superradiant decay of off-axis fluorescence is suppressed near
resonance due to attenuation and multiple-scattering effects.
