Bose-Einstein Condensate on a Persistent-Supercurrent Atom Chip
Hiromitsu Imai, Kensuke Inaba, Makoto Yamashita, and Tetsuya Mukai
Optical Science Laboratory
Ultra-cold atoms manipulated by electromagnetic field have been used widely for the applications of quantum information processing and quantum metrology. As a promising quantum device for full quantum control of an atomic state, a persistent-supercurrent atom chip allows us to trap atoms in extremely stable and tight potential. So far we have trapped atoms in a magnetic field of the persistent-supercurrent atom chip [1, 2]. In this year, we achieved 87Rb Bose-Einstein condensate (BEC) by radio frequency induced evaporative cooling. This result paves the way for the development of quantum devices such as quantum memories and atom interferometers.
As shown in Fig. 1, Rb atoms were trapped in a potential generated by the magnetic field of a persistent supercurrent in a closed loop circuit and a bias magnetic field. To verify the condensation, we employed the time-of flight (TOF) imaging technique, and measured the momentum distribution of atoms released from the trapping potential after the evaporative cooling. Figure 2 (a) and (b) show the TOF image after 15 ms and the cross-section along the dotted line. These images were taken when a final radio frequency of the evaporative cooling vf was 1.770 MHz. The broad distribution indicates that the atomic gas remained in the thermal state. In Fig. 2 (c) and (d), a bimodal momentum distribution was observed as the onset of the BEC transition at vf = 1.750 MHz. When vf was further decreased to 1.735 MHz [Figure 2 (e) and (f)], the thermal distribution was not observed and an almost pure condensate with a narrow distribution was obtained.
This work was supported by FIRST program, KAKENHI, and JST-CREST.
 T. Mukai et al., Phys. Rev. Lett. 98 (2007) 260407.
 C. Hufnagel et al., Phys. Rev. A 79 (2009) 053641.
Fig. 1. Schematic diagram of atoms trapped on a persistent-supercurrent atom chip.
Fig. 2. TOF images after 15 ms (upper panels) and cross-sections of the optical density along the dotted lines (lower panels).
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