Optically induced coherent intersubband excitations and spin -entangled states in semiconductor nanostructures.

Abstract

Ultrafast laser pulses and coherent techniques are used to create coherent electronic excitations and spin-entangled states in semiconductor nanostructures. A microscopic theory of impulsive stimulated Raman scattering is presented. In a modulation doped GaAs quantum well, we demonstrate the generation and control of coherent intersubband charge-density and spin-density excitations, which are the collective modes of a two-dimensional-electron-gas. The energy difference between these modes arises from Coulomb and exchange interactions. We also demonstrate the generation of coherent intersubband-inter-Landau-level excitations using similar techniques while placing the GaAs quantum well in a tilted magnetic field. In a dilute magnetic CdTe quantum well, we demonstrate the generation of spin-entangled states involving three donor electrons. The entanglement relies on the exchange interaction between optically excited holes and impurities. A model for multi-spin entanglement is presented. The entanglement is generated using circularly polarized laser pulses. Its signature is the observation of overtones of donor spin-flips in the differential reflectivity of the probe pulse.