1. Institute for Theoretical Physics, University of Innsbruck, Technikerstr. 21a, A-6020 Innsbruck, Austria
2. Station Q Quantum Architectures and Computation Group, Microsoft Research, Redmond, WA 98052, USA
3. LIACS, Leiden University, Niels Bohrweg 1, 2333 CA Leiden, The Netherlands
4. Department of Philosophy, University of Konstanz, Konstanz 78457, Germany
5. Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria