Creation of a Bose-condensed gas of 87Rb by laser cooling

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Science  24 Nov 2017:
Vol. 358, Issue 6366, pp. 1078-1080
DOI: 10.1126/science.aan5614
  • Fig. 1 Experimental setup and procedure.

    (A) 87Rb atoms are trapped in a 2D lattice formed by two orthogonal retroreflected trapping beams at 1064 nm. The cooling light at 795 nm propagates along the magnetic field (z) and is σ-polarized. (B) Simplified atomic-level structure for dRSC. The Zeeman splitting between two magnetic sublevels is matched to the vibrational splitting in the tightly confined direction. (C) Release-and-retrap compression sequence used to increase the atomic density. Starting from a sparsely filled 2D lattice, we perform dRSC (I) and then switch off the Y trapping beam to compress the atoms along y in the X trapping beam (II). After a short thermalization time, we switch back to the 2D lattice with an increased occupation number per trap (III). The procedure is repeated for the X beam to compress the atoms into a small number of tubes (IV). A final dRSC in this system (V) then yields a condensate.

  • Fig. 2 Evidence of the formation of a Bose-condensed gas during optical cooling.

    A bimodal velocity distribution emerges during the final laser cooling stage, indicating macroscopic population of the ground state. (A to C) Observed optical depth (OD) along z after a ballistic expansion of 1.3 ms for cooling times of 5 ms (A), 20 ms (B), and 80 ms (C), averaged over 200 repetitions of the experiment. The red lines are Gaussian fits to the wings of the distributions, and green lines are quadratic fits to the remaining distribution in the center (19). Here the intensity of the trapping beams is ramped down in 400 μs—slowly compared with the axial trapping frequency but quickly with regard to the motion along z—to reduce the interaction energy. (D) Evolution of the total atom number N (blue) or atom number per lattice tube N1 (red) versus the phase space density Embedded Image during the sequence, with the steps labeled I to V as defined in Fig. 1B. The solid lines represent the dRSC process, and the dotted lines represent the spatial compression. Each cooling step enhances Embedded Image by one order of magnitude, then the release-and-retrap compression increases the peak occupation number N1 while slightly decreasing Embedded Image. (E) Velocity distribution along z for the same parameters as in (C), but observed for a 2D gas after releasing the atoms into the 1D lattice Y.

  • Fig. 3 Condensate fraction.

    (A) Cooling performance in the final stage. Average kinetic energy measured by the time-of-flight method in the directions of strong (blue) and weak (red) confinement is plotted against cooling time. After 80 ms of cooling, the atoms are in the 2D vibrational ground state. The inset shows that there is almost no atom loss during cooling. (B) Approximate condensate fraction versus cooling time, derived from the bimodal velocity distribution. In (A) and (B), the dashed lines are exponential fits. (C) Approximate condensate fraction versus the inverse peak phase space density Embedded Image. The blue line is a theoretical prediction for an ideal gas of 50 atoms in a 1D trap (32). Error bars, SEM.

Supplementary Materials

  • Creation of a Bose-condensed gas of 87Rb by laser cooling

    Jiazhong Hu, Alban Urvoy, Zachary Vendeiro, Valentin Crépel, Wenlan Chen, Vladan Vuletić

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Figs. S1 to S4
    • Table S1
    • References

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