Read-out of single spins by optical spectroscopy

We review recent experiments on optical detection and manipulation of spin states of impurity centres in a solid. This research topic is of particular interest because of possible applications of the single-spin read-out technique in solid state quantum computing. The experimental background of optical detection of single quantum systems in solids is introduced. The interaction of an impurity centre with an excitation field is analysed in terms of optical Bloch equations. Recent experiments on the electron spin resonance of single organic molecules and paramagnetic defect centres in diamonds are presented. We show how the spin state of the nitrogen-vacancy paramagnetic defect in diamond can be read out optically. Pulsed electron spin resonance of the single paramagnetic defect centre in diamond is discussed.