[1] Karlsruher Inst Technol, Inst Phys, D-76131 Karlsruhe, Germany
[2] Univ Tubingen, Inst Phys, D-72076 Tubingen, Germany
[3] Univ Tubingen, Ctr Collect Quantum Phenomena, D-72076 Tubingen, Germany
[4] Karlsruher Inst Technol, Inst Mikro & Nanoelekt Syst, D-76187 Karlsruhe, Germany
[5] Johannes Gutenberg Univ Mainz, QUANTUM, Inst Phys, D-55128 Mainz, Germany