1. Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
2. National Institute of Standards and Technology, Boulder, Colorado 80305, USA
3. JILA, University of Colorado, Boulder, Colorado 80309, USA
4. Boulder Cryogenic Quantum Testbed, University of Colorado, Boulder, Colorado 80309, USA
5. HRL Laboratories, Malibu, California 90265, USA
6. Google, Inc., Mountain View, California 94043, USA