  When an off-resonant light field is coupled with atomic spins, its
polarization can rotate depending on the direction of the spins via a Faraday
rotation which has been used for monitoring and controlling the atomic spins.
We observed Faraday rotation by an angle of more than 10 degrees for a single
1/2 nuclear spin of 171Yb atom in a high-finesse optical cavity. By employing
the coupling between the single nuclear spin and a photon, we have also
demonstrated that the spin can be projected or weakly measured through the
projection of the transmitted single ancillary photon.
