Affiliation: | 1. Optics Section, The Blackett Laboratory , Imperial College , London, SW7 2BZ, England;2. Institute of Physics , Slovak Academy of Sciences , Dúbravská cesta 9, 842 28, Bratislava, Czechoslovakia;3. Max-Planck-Institüt für Quantenoptik , D-8046, Garching bei München, Germany |