  We investigate the effect of many-body interactions on the optical absorption
spectrum of a charge-tunable quantum dot coupled to a degenerate electron gas.
A constructive Fano interference between an indirect path, associated with an
intra dot exciton generation followed by tunneling, and a direct path,
associated with the ionization of a valence-band quantum dot electron, ensures
the visibility of the ensuing Fermi-edge singularity despite weak absorption
strength. We find good agreement between experiment and renormalization group
theory, but only when we generalize the Anderson impurity model to include a
static hole and a dynamic dot-electron scattering potential. The latter
highlights the fact that an optically active dot acts as a tunable quantum
impurity, enabling the investigation of a new dynamic regime of Fermi-edge
physics.
