Three-dimensional theory of quantum memories based on Λ-type atomic ensembles

We develop a three-dimensional theory for quantum memories based on light storage in ensembles of Lambda-type atoms, where two long-lived atomic ground states are employed. We consider light storage in an ensemble of finite spatial extent and we show that within the paraxial approximation the Fresnel number of the atomic ensemble and the optical depth are the only important physical parameters determining the quality of the quantum memory. We analyze the influence of these parameters on the storage of light followed by either forward or backward read-out from the quantum memory. We show that for small Fresnel numbers, the forward memory provides higher efficiencies, whereas for large Fresnel numbers, the backward memory is advantageous. The optimal light modes to store in the memory are presented together with the corresponding spin-waves and outcoming light modes. We show that for high optical depths such Lambda-type atomic ensembles allow for highly efficient backward and forward memories even for small Fresnel numbers $F\gtrsim 0.1$.Comment: Final version, 19 pages, 11 figure

Phonon-assisted relaxation and tunneling in self-assembled quantum dot molecules

We study theoretically phonon-assisted relaxation processes in a system consisting of one or two electrons confined in two vertically stacked self-assembled quantum dots. The calculation is based on a k.p approximation for single particle wave functions in a strained self-assembled structure. From these, two-particle states are calculated by including the Coulomb interaction and the transition rates between the lowest energy eigenstates are derived. We take into account phonon couplings via deformation potential and piezoelectric interaction and show that they both can play a dominant role in different parameter regimes. Within the Fermi golden rule approximation, we calculate the relaxation rates between the lowest energy eigenstates which lead to thermalization on a picosecond time scale in a narrow range of dot sizes.Comment: 13 pages, 10 figures; moderately modified, some new dscussion (including 1 new figure