  The interaction between a single confined spin and the spins of a Fermionic
reservoir leads to one of the most spectacular phenomena of many body physics
-- the Kondo effect. Here we report the observation of Kondo correlations in
optical absorption measurements on a single semiconductor quantum dot
tunnel-coupled to a degenerate electron gas. In stark contrast to transport
experiments, absorption of a single photon leads to an abrupt change in the
system Hamiltonian and a quantum quench of Kondo correlations. By inferring the
characteristic power law exponents from the experimental absorption
line-shapes, we find a unique signature of the quench in the form of an
Anderson orthogonality catastrophe, originating from a vanishing overlap
between the initial and final many-body wave-functions. We also show that the
power-law exponents that determine the degree of orthogonality can be tuned by
applying an external magnetic field which gradually turns the Kondo
correlations off. Our experiments demonstrate that optical measurements on
single artificial atoms offer new perspectives on many-body phenomena
previously studied exclusively using transport spectroscopy. Moreover, they
initiate a new paradigm for quantum optics where many-body physics influences
electric field and intensity correlations.
