  We report a time-resolved study of collective emission in dense ensembles of
two-level atoms. We compare, on the same sample, the build-up of superradiance
and subradiance from the ensemble when driven by a strong laser. This allows us
to measure the dynamics of the population of superradiant and subradiant states
as a function of time. In particular we demonstrate the build up in time of
subradiant states through the superradiant dynamics. This illustrates the
dynamics of the many-body density matrix of superradiant ensembles of two-level
atoms when departing from the ideal conditions of Dicke superradiance in which
symmetry forbids the population of subradiant states.
