  A quantum-mechanical many-particle system may exhibit non-local behavior in
that measurements performed on one of the particles can affect a second one
that is far apart. These so-called entangled states are crucial for the
implementation of quantum information protocols and gates for quantum
computation. Here, we use ultrafast optical pulses and coherent pump-probe
techniques to create and control spin entangled states in an ensemble of up to
three non-interacting electrons bound to donors in a Cd1-xMnxTe quantum well.
Our method, relying on the exchange interaction between optically-excited
excitons and the paramagnetic impurities, can in principle be applied to
entangle an arbitrarily large number of electrons. A microscopic theory of
impulsive stimulated Raman scattering and a model for multi-spin entanglement
are presented. The signature of entanglement is the observation of overtones of
donor spin-flips in the differential reflectivity of the probe pulse. Results
are shown for resonant excitation of localized excitons below the gap, and
above the gap where the signatures of entanglement are significantly enhanced.
Data is also presented on the generation of coherent excitations of
antiferromagnetically-coupled manganese pairs, folded acoustic phonons, exciton
Zeeman beats and entanglement involving two Mn2+ ions.
