  The electron spin coherence in n-doped and undoped, self-assembled
CdSe/Zn(S,Se) quantum dots has been studied by time-resolved pump-probe Kerr
rotation. Long-lived spin coherence persisting up to 13 ns after spin
orientation has been found in the n-doped quantum dots, outlasting
significantly the lifetimes of charge neutral and negatively charged excitons
of 350 - 530 ps. The electron spin dephasing time as long as 5.6 ns has been
measured in a magnetic field of 0.25 T. Hyperfine interaction of resident
electrons with a nuclear spin fluctuations is suggested as the main limiting
factor for the dephasing time. The efficiency of this mechanism in II-VI and
III-V quantum dots is analyzed.
