1. JILA, University of Colorado, Boulder and National Institute of Standards and Technology, 440 UCB, Boulder, Colorado 80309, USA
2. Department of Chemistry, University of Colorado, Boulder, 215 UCB, Boulder, Colorado 80309, USA
3. Department of Physics, University of Colorado, Boulder, 390 UCB, Boulder, Colorado, 80309, USA
4. Department of Applied Physics, Stanford University, 348 Via Pueblo Mall, Stanford, CA 94305-4090, USA
5. Department of Physics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan