Laser Cooling to Quantum Degeneracy

We report on Bose-Einstein condensation in a gas of strontium atoms, using laser cooling as the only cooling mechanism. The condensate is formed within a sample that is continuously Doppler cooled to below $1\mu \mathrm{K}$ on a narrow-linewidth transition. The critical phase-space density for condensation is reached in a central region of the sample, in which atoms are rendered transparent for laser cooling photons. The density in this region is enhanced by an additional dipole trap potential. Thermal equilibrium between the gas in this central region and the surrounding laser cooled part of the cloud is established by elastic collisions. Condensates of up to ${10}^5$ atoms can be repeatedly formed on a time scale of 100 ms, with prospects for the generation of a continuous atom laser.

Figure 1

Scheme to reach quantum degeneracy by laser cooling. (a) A cloud of atoms is confined in a deep reservoir dipole trap and exposed to a single laser cooling beam (red arrow). Atoms are rendered transparent by a “transparency” laser beam (green arrow) and accumulate in a dimple dipole trap by elastic collisions. (b) Level scheme showing the laser cooling transition and the transparency transition. (c) Potential experienced by ${}^{1}S_0$ ground-state atoms and atoms excited to the ${}^{3}P_1$ state. The transparency laser induces a light shift on the ${}^{3}P_1$ state, which tunes the atoms out of resonance with laser cooling photons. (d)–(f) Absorption images of the atomic cloud recorded using the laser cooling transition. The images show the cloud from above and demonstrate the effect of the transparency laser (e) and the dimple (f). (d) is a reference image without these two laser beams.

Figure 2

Creation of a BEC by laser cooling. Shown are time-of-flight absorption images and integrated density profiles of the atomic cloud for different times $t$ after the transparency laser has been switched on, recorded after 24 ms of free expansion. (a), (b) The appearance of an elliptic core at $t=160\mathrm{ms}$ indicates the creation of a BEC. (c) Same as in (b), but to increase the visibility of the BEC, atoms in the reservoir trap were removed before the image was taken. The fits (blue lines) consist of Gaussian distributions to describe the thermal background and an integrated Thomas-Fermi distribution describing the BEC. The red lines show the component of the fit corresponding to the thermal background. The $x^{\prime }{y}^{\prime }$ plane is rotated by 45° around the $z$ axis with respect to the $xy$ plane and the field of view of the absorption images is $2\mathrm{mm}\times 1.4\mathrm{mm}$.

Figure 3

Characterization of the BEC formation process after the transparency laser is switched on. The evolution of the atom number in the dimple and the reservoir (a), the evolution of temperature in these regions (b), and the BEC atom number (c), are shown. During the first 10 ms of this evolution, the dimple trap is ramped on. After 60 ms a BEC is detected.

Figure 4

Repeated destruction and reformation of the BEC. Shown is the evolution of BEC atom number while the BEC is destroyed every 200 ms (arrows) by suddenly increasing the depth of the dimple trap. If the system is laser cooled, the BEC atom number quickly increases again, which is shown here for up to 18 cycles (filled red circles). Without laser cooling, a BEC is detectable for at most five heating cycles, of which the first three are shown here (open black squares). Sample error bars indicate the statistical errors of three experimental realizations.