  We have employed the 12.6 GHz microwave transition resonance of a single
trapped$^{171}$Yb+ ion to accurately measure the size and relative orientation
of the magnetic and optical electric fields at the position of the ion in the
trap. Accurate knowledge of these fields is required for precision experiments
such as single ion PNC. As a proof of the principle we have measured the
polarization dependent light-shift of the ground state hyperfine levels due to
the 369 nm cooling laser to determine its electric field amplitude and
polarization.
