[1] Univ Innsbruck, Inst Theoret Phys, A-6020 Innsbruck, Austria
[2] Austrian Acad Sci, Inst Quantum Opt & Quantum Informat, A-6020 Innsbruck, Austria
[3] Scuola Super Meridionale, Largo San Marcellino 10, I-80138 Naples, Italy
[4] Univ Trieste, Dept Phys, Str Costiera 11, I-34151 Trieste, Italy
[5] Ist Nazl Fis Nucl, Trieste Sect, Via Valerio 2, I-34127 Trieste, Italy