1. JARA-FIT Institute for Quantum Information, Forschungszentrum Jülich GmbH and RWTH Aachen University, D 52074 Aachen, Germany
2. Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria
3. IHP, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany