  We investigate the possibilities of preserving and manipulating the coherence
of atomic two-level systems by ideal projective homodyne detection and
feedback. For this purpose, the photon emission process is described on time
scales much shorter than the lifetime of the excited state using a model based
on Wigner-Weisskopf theory. The backaction of this emission process is
analytically described as a quantum diffusion of the Bloch vector. It is shown
that the evolution of the atomic wavefunction can be controlled completely
using the results of homodyne detection. This allows the stabilization of a
known quantum state or the creation of coherent states by a feedback mechanism.
However, the feedback mechanism can never compensate the dissipative effects of
quantum fluctuations even though the coherent state of the system is known at
all times.
