We construct Lindbladians associated with controlled stochastic Hamiltonians in weak coupling. This allows to determine the power spectrum of the noise from measurements of dephasing rates; to optimize the control and to test numerical algorithms tha
t solve controlled stochastic Schrodinger equations. A few examples are worked out in detail.
We demonstrate control over the spin state of a semiconductor quantum dot exciton using a polarized picosecond laser pulse slightly detuned from a biexciton resonance. The control pulse follows an earlier pulse, which generates an exciton and initial
izes its spin state as a coherent superposition of its two non-degenerate eigenstates. The control pulse preferentially couples one component of the exciton state to the biexciton state, thereby rotating the excitons spin direction. We detect the rotation by measuring the polarization of the exciton spectral line as a function of the time-difference between the two pulses. We show experimentally and theoretically how the angle of rotation depends on the detuning of the second pulse from the biexciton resonance.
