  We analyzed the photoluminescence intermittency generated by a single
paramagnetic spin localized in an individual semiconductor quantum dot. The
statistics of the photons emitted by the quantum dot reflect the quantum
fluctuations of the localized spin interacting with the injected carriers.
Photon correlation measurements which are reported here reveal unique
signatures of these fluctuations. A phenomenological model is proposed to
quantitatively describe these observations, allowing a measurement of the spin
dynamics of an individual magnetic atom at zero magnetic field. These results
demonstrate the existence of an efficient spin relaxation channel arising from
a spin-exchange with individual carriers surrounding the quantum dot. A
theoretical description of a spin-flip mechanism involving spin exchange with
surrounding carriers gives relaxation times in good agreement with the measured
dynamics.
