Spin-Singlet Bose-Einstein Condensation of Two-Electron Atoms

We report the observation of a Bose-Einstein condensation of ytterbium atoms by evaporative cooling in a novel crossed optical trap. Unlike the previously observed condensates, a ytterbium condensate is a two-electron system in a singlet state and has distinct features such as the extremely narrow intercombination transitions which are ideal for future optical frequency standard and the insensitivity to external magnetic field which is important for precision coherent atom optics, and the existence of the novel metastable triplet states generated by optical excitation from the singlet state.

Figure 1

Potential depths induced by (a) the horizontal FORT beam and (b) the vertical FORT beam, which are calculated from the beam waists and the peak intensities of trap beams, as a function of time in evaporative cooling. It is to be noted that the vertical gradient of the vertical FORT beam is too small to support atoms against the gravity field, and thus the trap depth along the vertical direction is mostly determined by the horizontal beam. The evaporative cooling is carried out by gradually decreasing the potential depth due to the horizontal FORT beam from 630 to $2.2\mu \mathrm{K}$ in 2 s while keeping the potential due to the vertical FORT beam constant of $53\mu \mathrm{K}$.

Figure 2

Pseudocolor absorption images of ${}^{174}\mathrm{Y}\mathrm{b}$ atoms as a function of time after release from the trap whose trap depths are (a) $2.2\mu \mathrm{K}$ and (b) $6.7\mu \mathrm{K}$, respectively. In (a) the vertical widths remain nearly constant while the horizontal widths increase rapidly because of anisotropic trap confinement. In contrast, atoms are expanded isotropically in (b). The temperature is $0.9\mu \mathrm{K}$.

Figure 3

Density profiles of three sample of ${}^{174}\mathrm{Y}\mathrm{b}$ along the vertical direction after 5 ms of free expansion, showing transition to BEC. The corresponding pseudocolor absorption images are also shown. The profiles are obtained by integrating the column atom density distribution along the horizontal axis. The red broken lines show the best Gaussian fits to the wings: (a) pure condensate; (b) mixture of condensate and thermal cloud, $T=0.46\mu \mathrm{K}$; (c) pure thermal cloud, $T=0.79\mu \mathrm{K}$.

Figure 4

Measured horizontal widths of condensate whose images are shown in Fig. 2a, as a function of expanding time. The best fit is also shown as a solid line. The obtained chemical potential is $0.55\mu \mathrm{K}$.