[1] Univ Innsbruck, Inst Theoret Phys, Technikerstr 21a, A-6020 Innsbruck, Austria
[2] Univ Basel, Dept Phys, Klingelbergstr 82, CH-4056 Basel, Switzerland
[3] Max Planck Inst Quantum Opt, Hans Kopfermann Str 1, D-85748 Garching, Germany
[4] Univ Konstanz, Fachbereich Philosophie, Univ Str 10, D-78464 Constance, Germany