Precise control of electronic spins in semiconductors should lead to development of novel electronic systems based on the carriers' spin degree of freedom. Magnetic semiconductor quantum dots (QDs), where excitons can interact strongly with the magnetic atoms, hold particular promise as building blocks for such spin-based systems. This requires the ability to detect and manipulate individual spins. We will show in this presentation how we can optically probe the magnetic state of a single Mn atom embedded in an individual QD.
In the case of a quantum dot incorporating a single magnetic atom (spin S) and a single confined exciton, the exchange interaction between the exciton and the magnetic atom acts as an effective magnetic field, so that the atom's spin levels are split even in the absence of any applied magnetic field [1,2]. A set of (2S + 1) discrete emission lines can be resolved in magneto-optic micro-spectroscopy experiments, providing a direct view of the atom's spin states at the instant when the exciton annihilates.
The influence of the number of confined carriers on the spin splitting will be then considered by investigating both the biexciton and trion (X-) transitions in the same Mn-doped QD. The injection of the second electron-hole pair cancels the exchange interaction with the Mn ion and the spin degeneracy is almost restored. Bias controlled single carrier charging allows us to tune the presence of excess carriers in the dots. The fine structure of charged excitons coupled with a single Mn spin differs strongly from the exciton-Mn one . This can be attributed to the absence of electron-hole exchange interaction in the case of charged exciton.
 L. Besombes et al. Phys. Rev. Lett. 93, 207403 (2004)
 Y. Léger et al. Phys. Rev. Lett. 95, 047403 (2005)
 Y. Léger et al. to be published in PRL (2006)