1. Institute for Quantum Optics and Quantum Information – IQOQI Vienna, Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria
2. Atominstitut, Technische Universität Wien, Stadionallee 2, 1020 Vienna, Austria
3. present address: Quantum Technology Laboratories GmbH, Clemens-Holzmeister-Straße 6/6, 1100 Vienna, Austria