Continuous Loading of a Conservative Potential Trap from an Atomic Beam

We demonstrate the fast accumulation of ${}^{52}\mathrm{Cr}$ atoms in a conservative potential from a guided atomic beam. Without laser cooling on a cycling transition, a dissipative step involving optical pumping allows us to load atoms at a rate of $2\times {10}^7{\mathrm{s}}^{-1}$ in the trap. Within less than 100 ms we reach the collisionally dense regime, from which we produce a Bose-Einstein condensate with subsequent evaporative cooling. This constitutes a new approach to degeneracy where Bose-Einstein condensation can be reached without a closed cycling transition, provided that a slow beam of particles can be produced.

Figure 1

(a) Illustration of the dissipative loading scheme from a guided atom beam. A superposition of an ODT and a magnetic field confines the atoms radially ($xy$ plane). Along the $z$ direction the hybrid potential can be either repulsive or attractive depending on the atom’s magnetic sublevel. Arriving atoms are in the low-field seeking $m_J=3$ state. The magnetic field thus acts as a barrier. At the position of the barrier’s maximum, the atoms are pumped to the high-field seeking absolute ground state $m_J=-3$ while the directed kinetic energy is dissipated. (b) Magnetic substructure of the ${}^{7}S_3\to {}^{7}P_3$ transition in ${}^{52}\mathrm{Cr}$ used for optical pumping.

Figure 2

Illustration of the experimental realization. The atoms travel along the $z$ axis towards the focus of the ODT. The atomic beam is radially confined by the ODT laser beam. The magnetic field barrier is created by a pair of coils, fed by a current $I_{\mathrm{BC}}$. An optical pumping beam is passing through the center of the coils and pumps the atoms into the lowest Zeeman sublevel. Absorption images are taken by illuminating the cloud along the $y$ axis.

Figure 3

In trap absorption images at the position of the cloud. (a) Magnetic guide. A picture of the magnetically guided atomic beam. (b) Magnetic $\mathrm{\text{guide}}+\mathrm{ODT}$. Superimposing the ODT laser beam on axis with the magnetically guided atoms a large fraction of the atomic beam is funneled into the ODT potential. (c) Magnetic $\mathrm{\text{guide}}+\mathrm{ODT}+\mathrm{\text{magnetic}}$ barrier. Adding the magnetic barrier the atoms are slowed. Part of the atoms has enough energy to cross the barrier, the rest is reflected. (d) Magnetic $\mathrm{\text{guide}}+\mathrm{ODT}+\mathrm{\text{magnetic barrier}}+\mathrm{\text{optical}}$ pumping. Adding the optical pumping beam completes the dissipative loading mechanism. Atoms accumulate in the trap. (e) Fully loaded trap. After a loading time of 100 ms the atomic beam is magnetically shifted to the side, ending the loading process.

Figure 4

Evidence for collisions in the loaded ODT. The graphs show data obtained during trap loading and holding from in trap and time of flight absorption pictures. After loading times $t_{\mathrm{load}}$ of 25, 50, and 100 ms, respectively, the loading process is stopped. (a) Number of trapped atoms as a function of loading and holding time. (b) Density as a function of time. (c) Phase space density as a function of time. (d)–(f) Evolution of the radial and longitudinal temperature $T_x$ and $T_z$, respectively, for different loading times $t_{\mathrm{load}}$. The shaded areas emphasizes the loading time. Insets in (a) and (f) have logarithmic $t$ axes to illustrate the behavior at short time scales.