  Coherent population trapping (CPT) provides a highly sensitive means for
probing the energy level structure of an atomic system. For a nitrogen vacancy
center in diamond, the CPT offers an alternative to the standard
optically-detected magnetic resonance method for measuring the hyperfine
structure of the electronic ground states. We show that the nuclear spin
dependent CPT measures directly the hyperfine splitting of these states due to
the 14N nuclear spin. The CPT spectral response obtained in the presence of a
strong microwave field, resonant or nearly resonant with a ground state spin
transition, maps out the dynamic Stark splitting induced by the coherent spin
excitation.
