[1] Swiss Fed Inst Technol, Inst Theoret Phys, CH-8093 Zurich, Switzerland
[2] CALTECH, Inst Quantum Informat & Matter, Pasadena, CA 91125 USA
[3] Univ Strathclyde, Dept Phys, Glasgow G4 0NG, Lanark, Scotland
[4] Univ Strathclyde, SUPA, Glasgow G4 0NG, Lanark, Scotland