[1] Albert Ludwigs Univ Freiburg, Phys Inst, Hermann Herder Str 3, D-79104 Freiburg, Germany
[2] Univ Parma, Dipartimento Sci Matemat Fis & Informat, Campus Univ,Parco Area Sci 7-A, I-43124 Parma, Italy
[3] Ist Nazl Fis Nucl, Sez Milano Bicocca, Grp Collegato Parma, I-43124 Parma, Italy
[4] Albert Ludwigs Univ Freiburg, Freiburg Inst Adv Studies, Albertstr 19, D-79104 Freiburg, Germany
[5] EUCOR Ctr Quantum Sci & Quantum Comp, Hermann Herder Str 3, D-79104 Freiburg, Germany
[6] Univ Salamanca, Dept Fis Fundamental, E-37008 Salamanca, Spain