1. Group of Applied Physics, University of Geneva, Chemin de Pinchat 22, CH-1211 Geneva 4 Switzerland
2. Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel Switzerland
3. Department of Quantum Matter Physics, University of Geneva, 24, Quai Ernest-Ansermet, CH-1211 Geneva 4 Switzerland
4. Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom