1. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
2. Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
3. Department of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
4. Center for Advanced Materials, Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA