[1] Univ Stuttgart, Inst Phys 3, D-70569 Stuttgart, Germany
[2] Ruhr Univ Bochum, Angew Festkorperphys, D-44780 Bochum, Germany
[3] Univ Ulm, Inst Quantenopt, D-89081 Ulm, Germany
[4] Univ Waterloo, Inst Quantum Comp, Waterloo, ON N2L 3G1, Canada
[5] Univ Augsburg, Expt Phys 4, D-86159 Augsburg, Germany
[6] Ruhr Univ Bochum, RUBION, D-44780 Bochum, Germany