Bose-Einstein condensation of erbium atoms in a quasielectrostatic optical dipole trap

Quantum gases of rare-earth elements are of interest due to the large magnetic moment of many of those elements, leading to strong dipole-dipole interactions, as well as an often nonvanishing orbital angular momentum in the electronic ground state, with prospects for long coherence time Raman manipulation, and state-dependent lattice potentials. We report on the realization of a Bose-Einstein condensate of erbium atoms in a quasielectrostatic optical dipole trap generated by a tightly focused midinfrared optical beam derived from a ${\mathrm{CO}}_2$ laser near $10.6\mu \mathrm{m}$ in wavelength. The quasistatic dipole trap is loaded from a magneto-optic trap operating on a narrow-line erbium laser cooling transition near 583 nm in wavelength. Evaporative cooling within the dipole trap takes place in the presence of a magnetic field gradient to enhance the evaporative speed, and we produce spin-polarized erbium Bose-Einstein condensates with $3\times {10}^4$ atoms.

Figure 1

Erbium atomic level scheme. Shown is the energetic position (in units of $hc$, the Planck constant $h$ and the speed of light $c)$ of levels versus the total angular momentum $J$, where levels of negative parity are shown as red (gray) lines [16]. For operation of the Zeeman slower, the strong cooling transition near 401 nm is used, while the magneto-optic trap uses optical beams tuned to near the weaker electronic transition near 583 nm in wavelength. For dipole trapping of atoms, radiation near $10.6\mu \mathrm{m}$ emitted by a ${\mathrm{CO}}_2$ laser is used.

Figure 2

(a) Schematic of the used experimental setup and (b) time sequence used in the experimental operation. A magneto-optic trap collects cold erbium atoms from a Zeeman-slowed atomic beam. We then switch to a compressed magneto-optic trap phase, from which the quasistatic dipole trap is loaded. Atoms confined in the optical dipole trap are then evaporatively cooled to quantum degeneracy, after which the atomic cloud is analyzed with absorption imaging.

Figure 3

Optical column density (vertically shifted) of cold erbium atomic clouds versus transverse position, as measured with a weak optical probe pulse tuned to the 401-nm erbium transition, for several values of the final trapping beam power. As the depth of the dipole trap decreases (values for trap depth refer to calculated values in units of the Boltzmann constant, $k_B)$, the distribution becomes bimodal, corresponding to a Bose-Einstein-condensed peak on top of a broad thermal cloud. The topmost plot gives data for an almost pure Bose-Einstein condensate.

Figure 4

(a) Shadow images of the atomic erbium clouds for different expansion times after release from the dipole trap. The color bar shows the optical column density in arbitrary units. (b) Variation of the radial (blue dots) and longitudinal (green squares) $1/e^2$ width versus the time of free flight, showing the expansion of the atomic cloud upon release from the trap. At approximately 16-ms free expansion time, the aspect ratio inverts.

Figure 5

Measured lifetime of atomic clouds confined in the quasistatic optical dipole trap versus the condensate fraction. The leftmost data point was recorded for a thermal cloud with temperature slightly above the BEC transition temperature. As the condensate fraction increases, the observed lifetime reduces.