[1] Univ Innsbruck, Inst Theoret Phys, A-6020 Innsbruck, Austria
[2] Austrian Acad Sci, Inst Quantum Opt & Quantum Informat, A-6020 Innsbruck, Austria
[3] Harvard Smithsonian Ctr Astrophys, ITAMP, Cambridge, MA 02138 USA
[4] Harvard Univ, Dept Phys, Cambridge, MA 02138 USA