Entanglement and control of quantum states

In the first part of this thesis,we examine the preparation of a
single- mode radiation field in arbitrary pure quantum states via
resonant inter- action with a sequence of two-level atoms.The
preparation is achieved by choosing an appropriate (in general
entangled)initial state of the atomic sequence,and does neither
require a final state measurement of the atoms,nor a control of the
atom-field interaction.Furthermore,the method is applicable also
when starting from mixed initial field states. We show how to
determine the optimal initial atomic state which pre- pares the
desired field state with the maximum fidelity,and prove the
feasibility of our state preparation method by numerical
calculations. In the second part,we demonstrate the noise-induced
control of quantum jumps in a fundamental open quantum
system.Here,in addition to the subsequent interaction with a flux of
two-level atoms,the quantized field is also coupled to a thermal
environment.Under certain experimental conditions,the photon field
exhibits a bistable behavior,with quantum jumps between two
metastable states.In the presence of a small peri- odic signal
(i.e.,a modulation of the initial state of the two-level atoms
crossing the single-mode resonator),the best synchronization of
these quantum jumps with the signal is achieved at an
optimal,nonvanishing temperature of the environment.This stochastic
resonance e ffect can be observed in di fferent components of the
atomic Bloch vector on exit from the cavity. The third part treats
a speci fic problem concerning the characterization of entanglement
between two quantum mechanical two-level systems. We consider the
optimal decomposition of a two-qubit state into an en- tangled and
a separable part,with maximal weight of the latter,and derive
necessary and su fficient conditions for the optimality of the de-
composition.