Ultrafast optical rotations of electron spins in an InGaAs/GaAs quantum dot ensemble

We report on fast optical rotation operations on electron spins in a quantum dots ensemble. The spins are initialized in the z direction (quantum dot growth and light propagation direction). The spin vector oscillates about a transversal magnetic field B till a ultrafast 2π-''control'' laser pulse induces rotations of the spins about the z axis. The 2π-control pulse rotates the spin without generating a new spin polarization. The rotation angle is determined by the photon energy detuning of the control pulse from the optical resonance. For the first time for optically controlled spins, spin echoes and extension of the dephasing time were seen. By combining the rotation about the two axis a spin rotations about arbitrary axis has been realized. This robust optically controlled single spin rotation gate provides the basis for single-qubit logic operations.