Ballistic spin transport without net charge transport in quantum wells

We demonstrate the ballistic transport of electron spin in the absence of net transport of electron charge. Spin-up electrons move in one direction, while an equal number of spin-down electrons move with the same velocities in the opposite direction, yielding no net charge motion. This effect, also referred to as a ‘pure’ spin current, results from the quantum mechanical interference of the transition amplitudes associated with two-photon absorption of an optical field of frequency ω and one-photon absorption of a field of frequency 2ω. By changing the relative phase of ω and 2ω fields, we demonstrate coherent control of the magnitude and direction of this pure spin current. Results agree with recent theoretical predictions. We use polarization-sensitive pump–probe techniques to monitor the resulting ballistic spin transport with <10 nm spatial resolution.