  Photochromism in single nitrogen-vacancy optical centers in diamond is
demonstrated. Time-resolved optical spectroscopy shows that intense irradiation
at 514 nm switches the nitrogen-vacancy defects to the negative form. This
defect state relaxes back to the neutral form under dark conditions. Temporal
anticorrelation of photons emitted by the different charge states of the
optical center unambiguously indicates that the nitrogen-vacancy defect
accounts for both 575 nm and 638 nm emission bands. Possible mechanism of
photochromism involving nitrogen donors is discussed.
