1. Department of Physics and Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
2. The Abdus Salam International Centre for Theoretical Physics (ICTP), strada Costiera 11, 34151 Trieste, Italy
3. International School for Advanced Studies (SISSA), via Bonomea 265, 34136 Trieste, Italy
4. Department of Physics, University of Trieste, 34127 Trieste, Italy
5. Istituto Nazionale di Ottica del Consiglio Nazionale delle Ricerche (CNR-INO), 34149 Basovizza-Trieste, Italy