1. Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA
2. Department of Physics and Astronomy, and Stewart Blusson Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
3. Department of Electrical and Computer Engineering, Princeton University, Princeton, New Jersey 08544, USA
4. Department of Physics, University of Massachusetts, Amherst, Massachusetts 01003, USA