Quantum degenerate mixtures of strontium and rubidium atoms

We report on the realization of quantum degenerate gas mixtures of the alkaline-earth-metal element strontium with the alkali-metal element rubidium. A key ingredient of our scheme is sympathetic cooling of Rb by Sr atoms that are continuously laser cooled on a narrow-linewidth transition. This versatile technique allows us to produce ultracold gas mixtures with a phase-space density of up to 0.06 for both elements. By further evaporative cooling we create double Bose-Einstein condensates of ${}^{87}$Rb with either ${}^{88}$Sr or ${}^{84}$Sr, reaching more than $1\times {10}^5$ condensed atoms per element for the ${}^{84}$Sr-${}^{87}$Rb mixture. These quantum gas mixtures constitute an important step towards the production of a quantum gas of polar, open-shell RbSr molecules.

Figure 1

Timing of the experimental sequence and trap configurations used to produce a ${}^{88}$Sr-${}^{87}$Rb double BEC. (a) Timing sequence. The central sympathetic laser cooling of ${}^{87}$Rb by ${}^{88}$Sr shaded in gray is characterized in Fig. 2. (b, c) Dipole trap configurations and atomic clouds (b) at the end of the preparation stage and (c) during the evaporation stage (not to scale).

Figure 2

Sympathetic laser cooling of ${}^{87}$Rb by ${}^{88}$Sr. During the compression phase the density of the red Sr MOT is increased and the Sr atoms are loaded into the storage dipole trap by changing magnetic field and both the MOT laser detuning and intensity. During the hold phase these parameters are held constant. Evolution of the Rb and Sr (a) atom numbers and (b) temperatures as determined from time-of-flight absorption images. During a part of the compression phase the Sr cloud separates into two components, one being heated out of the system, the other remaining trapped and being transferred into the storage trap. We only show the number of trapped Sr atoms and the Sr temperature if we can determine them reliably from bimodal fits to the absorption images. (c) PSD of Rb, calculated from trap parameters and Rb atom number and temperature.

Figure 3

${}^{88}$Sr-${}^{87}$Rb double BEC. The absorption images have been recorded after an expansion time of 26 ms. The lower panels show density profiles obtained by vertical integration of the absorption images. Bimodal fits consisting of a Gaussian and a Thomas-Fermi distribution are shown as red solid lines. The Gaussian part of the fit corresponds to the thermal fraction of the cloud and is shown as blue dotted line.

Figure 4

Timing of the experimental sequence and trap configurations used to produce a ${}^{84}$Sr-${}^{87}$Rb double BEC. (a) Timing sequence. ${}^{88}$Sr is used for sympathetic laser cooling of Rb and discarded afterwards. ${}^{84}$Sr, stored in a red MOT during sympathetic laser cooling, is transferred into the science dipole trap before evaporative cooling. Dipole trap configurations and atomic clouds (b) at the end of the preparation stage and (c) after removing ${}^{88}$Sr (not to scale).

Figure 5

Formation of a ${}^{84}$Sr-${}^{87}$Rb double BEC while Sr is evaporated, sympathetically cooling Rb. (a) Absorption images of the Rb and Sr clouds at time $t$ of the evaporative cooling ramp recorded after 24 ms of expansion. Strontium condenses at $t=4.3$ s and Rb at $t=6$ s. The temperature $T$ of Sr is given. (b) Density profiles obtained from the absorption images at $t=6.8$ s by integration along the vertical direction. The solid red lines are bimodal fits to the data by a Gaussian plus a Thomas-Fermi distribution. The blue dotted line shows the Gaussian part of the fit corresponding to the thermal part of the cloud.