  We present a study of the nonlinear intersubband (ISB) response of conduction
electrons in a GaAs/AlGaAs quantum well to strong THz radiation, using a
density-matrix approach combined with time-dependent density-functional theory.
We demonstrate coherent control of ISB optical bistability, using THz control
pulses to induce picosecond switching between the bistable states. The
switching speed is determined by the ISB relaxation and decoherence times, T1
and T2.
