1. Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom
2. Colby College, 4000 Mayflower Hill Dr., Waterville, Maine 04901, USA
3. Department of Physics and Arnold Sommerfeld Center for Theoretical Physics, Ludwig-Maximilians-Universität München, Theresienstraße 37, 80333 Munich, Germany
4. Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore 117543