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We investigate the possibilities of preserving and manipulating the coherenceof atomic two-level systems by ideal projective homodyne detection andfeedback. For this purpose, the photon emission process is described on timescales much shorter than the lifetime of the excited state using a model basedon Wigner-Weisskopf theory. The backaction of this emission process isanalytically described as a quantum diffusion of the Bloch vector. It is shownthat the evolution of the atomic wavefunction can be controlled completelyusing the results of homodyne detection. This allows the stabilization of aknown quantum state or the creation of coherent states by a feedback mechanism.However, the feedback mechanism can never compensate the dissipative effects ofquantum fluctuations even though the coherent state of the system is known atall times.Comment: 12 pages RevTex and 7 figures, to be published in Phys. Rev. A, final  versio

Quantum control of atomic systems by homodyne detection and feedback