1. Department of Physics and Astronomy, Michigan State University 1 , East Lansing, Michigan 48824, USA
2. Department of Physics, Eidgenössiche Technische Hochschule Zürich 2 , 8093 Zürich, Switzerland
3. Quantum Center, Eidgenössiche Technische Hochschule Zürich 3 , 8093 Zürich, Switzerland