[1] Ludwig Maximilians Univ Munchen, Coulombwall 1, D-85748 Garching, Germany
[2] Max Planck Inst Quantum Opt, Hans Kopfermann Str 1, D-85748 Garching, Germany
[3] Univ Konstanz, Fachbereich Phys, Univ Str 10, D-78467 Constance, Germany
[4] Max Planck Gesell, Fritz Haber Inst, Faradayweg 4-6, D-14195 Berlin, Germany
[5] Stanford Univ, Dept Appl Phys, 348 Via Pueblo Mall, Stanford, CA 94305 USA