Quantum Degenerate Gases of Ytterbium Atoms

Yoshiro Takahashi

Department of Physics, Graduate School of Science, Kyoto University, Japan yitk@scphys.kyoto-u.ac.jp

Abstract

We report on the recent experiments aiming at the realization of various quantum degenerate gases such as Bose-Einstein condensates, Fermi degeneracy, and the mixture using ytterbium atoms.

Among many quantum degenerate gases, those of ytterbium (Yb) atoms1) are expected to present unique possibilities for fundamental researches and applications. The two valence electrons result in singlet and triplet states connected by extremely narrow intercombination transitions which have received considerable attention as a frequency standard with unprecedented precision2). The simple structure of the spinless ground state with no nuclear spin makes the Bose-Einstein condensates (BEC) insensitive to external magnetic field, which is invaluable in precision coherent atom interferometry. Furthermore, the optical excitation of a BEC to the metastable triplet state offers interesting system of a novel metastable state BEC where the interaction between the atoms in the P-states are dominated by quadrupole-quadrupole interaction3). In addition, the existence of rich varieties of stable isotopes of five bosons (168Yb, 170Yb, 172Yb, 174Yb, and 176Yb) and two fermions (171Yb and 173Yb) will allow us to study various interesting quantum degenerate gases using Yb atoms. Here we report on the recent experiments aiming at the realization of various quantum degenerate gases such as Bose-Einstein condensates, Fermi degeneracy, and the mixture using Yb atoms.

Firstly, the efforts devoted to the realization of Fermi degenerate gas of 171Yb are described. Successful evaporative cooling result was obtained with the tight trap configuration of optical lattice. The experiment was based on a magneto-optical trap with the weakly allowed intercombination transition and a crossed FORT system. The 171Yb atoms in a thermal beam were slowed by Zeeman slower technique with strong

singlet transition (1S0 – 1P1, the wavelength is 399 nm, the linewidth is 29 MHz), and then loaded into the MOT with intercombination transition4) (1S0 – 3P1, the wavelength is 556 nm, the linewidth is 182 kHz). The horizontal optical lattice beam and the vertical FORT beam were generated from cw diode-pumped solid-state lasers with the wavelength of 532 nm5). The atoms initially trapped in the vertical FORT beam were adiabatically transfered into the selected sites in the horizontal optical lattice. After the evaporative cooling in the optical lattice, about 104 atoms were cooled to 1.8µK which almost equals to Fermi temperature TF. To further cool down the temperature, we performed sympathetic evaporative cooling of fermions 171Yb with bosonic 174Yb atoms. So far, we could decrease the 171Yb temperature to about 3µK which corresponds to 3TF, but the further cooling was prevented by the rapid decrease of 174Yb atom number. Next, we pursued the BECs of other bosonic isotopes of 172Yb and 176Yb. Since the scattering lengths are so sensitive to the highest bound-state energy in the interaction potential, these two isotopes can have quite different scattering lengths in spite of the small mass difference. To investigate the isotopic difference in the collisional behavior, exactly the same experimental sequence of evaporative cooling was applied for these two isotopes as that for the 174Yb. We found the clear difference in the behavior of the evaporative cooling. We expect the sympathetic evaporative cooling will be efficiently applied to generate mixtures of BECs.

The ability to control the atom-atom interaction is very important in the study of quantum degenerate gases. Unfortunately, widely used technique of magnetically tuned Feshbach resonance cannot be used for Yb atoms due to the lack of the hyperfine structure in the ground state. Instead, we plan to exploit optical Feshbach resonance technique6) to control the atom-atom interaction. For this purpose, we have searched appropriate photo-association resonances, instead of the previously observed resonances associated with 1S0-1P1 transition7). Here we have newly found 11 resonances associated with the weak spin-forbidden 1S0-3P1 transition, which are good

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candidates for this control. Unique energy structure of Yb atoms provides

another possibility of controlling the atom-atom interaction in the metastable 3P2 state by Feshbach-resonance like phenomenon. Toward this possibility, we have transferred the ground state 1S0 Yb atoms into the 3P2 state via the 3D2 state in the FORT. So far we succeeded in high-density optical trapping of the atoms in that state. The trap frequency and the temperature of the atoms in the 3P2 state could be measured. Investigation of the atom-atom interaction is now undertaken. In additiuon, our excitation scheme allowed us to efficiently transfer the atoms in the ground state 1S0 into the metastable 3P0 state, the behavior in which was also investigated.

This work has been performed in

collaboration with Y. Takasu, T. Fukuhara, S. Tojo, M. Kitagawa, S. Uetake, A. Yamaguchi, D. Hashimoto, and M. Kumakura. References 1. Y. Takasu et al., Phys. Rev. Lett. 91, 040404

(2003). 2. J. Hall, M. Zhu, and P. Bush, J. Opt. Soc. Am.

B6, 2194 (1989). 3. A. Derevianko, et al., Phys. Rev. Lett. 90,

063002 (2003). 4. T. Kuwamoto, K. Honda, Y. Takahashi, and T.

Yabuzaki, Phys. Rev. A 60, R745 (1999). 5. Y. Takasu et al., Phys. Rev. Lett. 90, 023003

(2003). 6. P. O. Fedichev, et al. Phys. Rev. Lett. 77. 2913

(1996). 7. Y. Takasu et al., Phys. Rev. Lett. 93, 123202

(2004).

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