Terahertz excitation and coherence effects of two-level donor systems in GaAs quantum dots

The confinement effects on the electronic and donor states in GaAs-(Ga, Al)As quantum dots, in the presence of an applied magnetic field, are studied. Using the optical Bloch equations with damping, we study the time evolution of the 1s, 2p-, and 2p+ states in the presence of an applied magnetic field and of a terahertz laser. We discuss the conditions to obtain a bound 2p+ state in contrast to the bulk situation, where the 2p+ state is resonant with the continuum states (in this case, decoherence may be a serious problem for practical realizations). We show that the pronounced confining effects of semiconductor quantum dots lead to conditions such that the 2p+ excited donor state may lie below the continuum, originating a favourable situation concerning the coherence time of the corresponding Rabi oscillations. We also calculate the electric dipole transition moment, discuss the photosignal, and the possible experimental conditions under which decoherence is weak and qubit operations are efficiently controlled