[1] Swiss Fed Inst Technol, Inst Quantum Elect, CH-8093 Zurich, Switzerland
[2] Univ Paris Cite, CNRS, Mat & Phenomenes Quant, F-75013 Paris, France
[3] Harvard Univ, Dept Phys, Cambridge, MA 02138 USA
[4] Univ Cambridge, Cavendish Lab, Cambridge CB3 0HE, England
[5] Missouri Univ Sci & Technol, Dept Phys, Rolla, MO 65409 USA
[6] Harvard & Smithsonian, Ctr Astrophys, ITAMP, Cambridge, MA 02138 USA