Magnetic field control of photon echo in the electron-trion system:   Shuffling of coherences between optically accessible and inaccessible states
  We report on magnetic field induced oscillations of the photon echo signal from negatively charged excitons in a CdTe/(Cd,Mg)Te semiconductor quantum well. The oscillatory signal is due to Larmor precession of the electron spin about a transverse magnetic field and depends sensitively on the polarization configuration of the exciting and refocusing pulses. The echo amplitude can be fully tuned from maximum down to zero depending on the time delay between the two pulses and the magnetic field strength. The results are explained in terms of the optical Bloch equations accounting for the spin level structure of electron and trion. 
L. Langer, S. V. Poltavtsev, I. A. Yugova, D. R. Yakovlev, G.   Karczewski, T. Wojtowicz, J. Kossut, I. A. Akimov and M. Bayer
cond-mat.mes-hall