[1] Barcelona Inst Sci & Technol, ICFO Inst Ciencies Foton, E-08860 Castelldefels, Barcelona, Spain
[2] ICREA, Passeig Ll Campanys 23, E-08010 Barcelona, Spain
[3] Ludwig Maximilians Univ Munchen, Fak Phys, Schellingstr 4, D-80799 Munich, Germany
[4] Max Planck Inst Quantum Opt, Hans Kopfermann Str 1, D-85748 Garching, Germany
[5] QuSoft, CWI, Sci Pk 123, NL-1098 XG Amsterdam, Netherlands
[6] Univ Amsterdam, Sci Pk 123, NL-1098 XG Amsterdam, Netherlands
[7] Univ Ulm, Albert Einstein Allee 11, D-89081 Ulm, Germany
[8] Ctr Integrated Quantum Sci & Technol IQST, Albert Einstein Allee 11, D-89081 Ulm, Germany
[9] Swiss Fed Inst Technol, Dept Phys, CH-8093 Zurich, Switzerland
[10] Free Univ Berlin, Dahlem Ctr Complex Quantum Syst, D-14195 Berlin, Germany
[11] CEA Saclay, Serv Phys Etat Condense, F-91191 Gif Sur Yvette, France
[12] Univ Geneva, Grp Appl Phys, CH-1211 Geneva 4, Switzerland
[13] Tech Univ Munich, Dept Chem, Lichtenbergstr 4, D-85747 Garching, Germany
[14] Univ Strathclyde, Dept Phys, SUPA, Glasgow G4 0NG, Lanark, Scotland
[15] Phys Tech Bundesanstalt, QUEST Inst Expt Quantum Metrol, D-38116 Braunschweig, Germany
[16] Leibniz Univ Hannover, Inst Quantenopt, D-30167 Hannover, Germany
[17] Univ Oxford, Dept Phys, Clarendon Lab, Oxford OX1 3PU, England
[18] Saarland Univ, Theoret Phys, D-66123 Saarbrucken, Germany