1. Photonics Laboratory, ETH Zürich, 8093 Zürich, Switzerland
2. Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090 Wien, Austria
3. Erwin Schrödinger International Institute for Mathematics and Physics, University of Vienna, Boltzmanngasse 9, 1090 Wien, Austria
4. ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona 08860, Spain