  Faraday rotation of a laser field induced by a single atom is demonstrated by
tightly focussing a linearly polarized laser beam onto a laser-cooled ion held
in a harmonic Paul trap. The polarization rotation signal is further used to
measure the phase-shift associated with
electromagnetically-induced-transparency and to demonstrate read-out of the
internal state on the qubit transition with a detection fidelity of 98 $\pm$
1%. These results have direct implications for single atom magnetometery and
dispersive read-out of atomic superpositions.
