Cooling of Sr to high phase-space density by laser and sympathetic cooling in isotopic mixtures

Based on an experimental study of two-body and three-body collisions in ultracold strontium samples, a novel optical-sympathetic cooling method in isotopic mixtures is demonstrated. Without evaporative cooling, a phase-space density of $6\times {10}^{-2}$ is obtained with a high spatial density that should allow us to overcome the difficulties encountered so far to reach quantum degeneracy for Sr atoms.

Figure 1

Nonexponential decay of the number of ${}^{86}\mathrm{Sr}$, trapped in the optical dipole trap due to 3-body recombination. Inset: the 3-body recombination rate constant is given by the slope of the natural log of the atom number with respect to ${\int }_0^t⟨n^2\left(t^{\prime }\right)⟩d{t}^{\prime }$ where $⟨n^2\left(t^{\prime }\right)⟩=1∕N\left(t^{\prime }\right){\int }_V{n}^3(\vec{r},t^{\prime })d^{3}r$ (see the text). The term $t∕\tau$ accounts for losses due to background collisions.

Figure 2

Dynamics of an optically trapped ${}^{88}\mathrm{Sr}$ cloud sympathetically cooled with laser cooled ${}^{86}\mathrm{Sr}$. Filled circles ●: ${}^{88}\mathrm{Sr}$ atom number. Open circles 엯: ${}^{86}\mathrm{Sr}$ atom number. The number of ${}^{88}\mathrm{Sr}$ atoms remains constant during the process, while ${}^{86}\mathrm{Sr}$ decays exponentially with a $80\mathrm{ms}$ time constant. Under optimized conditions, the temperature (triangles ▿) decreases with a $150\mathrm{ms}$ time constant and the mixture is always at thermal equilibrium.

Figure 3

Asymptotic temperature of the ${}^{88}\mathrm{Sr}$ cloud for different intensities of the ${}^{86}\mathrm{Sr}$ laser cooling beam on the ${}^{1}S_0\text{−}{}^{3}P_1$ transition $(I_{\mathrm{sat}}=3\mu \mathrm{W}∕{\mathrm{cm}}^2)$. The ${}^{88}\mathrm{Sr}$ atom number is $6\times {10}^5$.

Figure 4

Temperature and phase-space density of the ${}^{88}\mathrm{Sr}$ cloud sympathetically cooled with laser cooled ${}^{86}\mathrm{Sr}$, as a function of the ${}^{88}\mathrm{Sr}$ atom number.