1. Department of Applied Physics, Stanford University, Stanford, California 94305-4045, USA
2. Department of Physics, Western Michigan University, Kalamazoo, Michigan 49008-5252, USA
3. Department of Physics, Stanford University, Stanford, California 94305-4045, USA
4. Attocube Systems AG, Königinstraße 11A, 80539 Munich, Germany
5. Department of Physics, Cornell University, Cornell, Ithaca, New York 14853, USA
6. IBM Research Division, T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
7. Department of Physics, University of Colorado Denver, Denver, Colorado 80217-3364, USA
8. Kavli Institute at Cornell, Ithaca, New York 14853, USA
9. OcteVue, Hadley, Massachusetts 01035, USA