Collective single-mode precession of electron spins in an ensemble of singly charged (In,Ga)As/GaAs quantum dots

We show that the spins of all electrons in an ensemble of singly charged (In,Ga)As/GaAs quantum dots can be driven into a single mode of precession about a magnetic field. This regime is achieved by allowing only this single mode within the electron-spin precession spectrum of the ensemble to be synchronized with a train of periodic optical excitation pulses. Under this condition a nuclei-induced frequency focusing leads to a shift of almost all spin precession frequencies into the synchronized mode. The macroscopic magnetic moment of the electron spins that is created in this regime precesses free of dephasing induced by inhomogeneous distribution of g factors.