1. Institute for Quantum Electronics, ETH Zurich, Wolfgang-Pauli-Strasse 16, CH-8093 Zurich,
Switzerland
2. Department
of Physics, University of Basel, Klingelbergstrasse
82, CH-4056
Basel, Switzerland
3. Institute
of Energy Technology, ETH Zurich, Sonneggstrasse
3, CH-8092 Zurich, Switzerland