1. Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
2. T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA
3. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
4. Brookhaven National Laboratory, Upton, New York 11973, USA
5. National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305 0044, Japan