  Rabi nutations of a single electron spin in a single defect center have been
detected. The coherent evolution of the spin quantum state is followed via
optical detection of the spin state. Coherence times up to several microseconds
at room temperature have been measured. Optical readout of the spin states
leads to decoherence, equivalent to the so-called Zeno effect. Quantum beats
between electron spin transitions in a single spin Hahn echo experiment are
observed. A closer analysis reveals that beats also result from the hyperfine
coupling of the electron spin to a single 14N nuclear spin. The results are
analysed in terms of a density matrix approach of an electron spin interacting
with two oscillating fields.
