1. JARA-FIT and 2nd Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany
2. Peter Grünberg Institute (PGI-8/9), Forschungszentrum Jülich, 52425 Jülich, Germany
3. Institute for Quantum Information, RWTH Aachen University, 52056 Aachen, Germany
4. Institute of Science and Technology Austria, 3400 Klosterneuburg, Austria