All-optical realization of quantum degeneracy of Yb atoms

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BECs of two-electron atoms such as ytterbium (Yb) and alkali-earth atoms have distinct features such as the extremely narrow intercombination transitions which are ideal for future optical frequency standard and the insensitivity to external magnetic field which is important for precision coherent atom optics. Here we report the achievement of BEC of Yb atoms by all-optical means in a novel crossed far-off resonant trap (FORT). The evaporative cooling is carried out by gradually decreasing the horizontal FORT beam power in 2 sec., while keeping the vertical FORT beam power constant. When the final trap depth is 2.2 ,, K, anisotropic expansion after release from the trap has appeared. The number of atoms in the condensate is typically 5×10^3. The critical temperature is about 0.7 ,, K and the peak atom density is 7×10^<14> cm^<-3>. The lifetime of the condensate is about 500 ms, and we infer an upper bound for the three-body loss coefficient of 4×10^<-30> cm^6 s^<-1>.From the behavior of the condensate, we have estimated the scattering length of ^<174>Yb. The chemical potential of 0.55, K is obtained from the fit with the horizontally expanded widths at ten TOF times, which gives the scattering length of 6 nm with the uncertainty of -5 and +10 nm. By combining this result with our recent photoassociation experiment, we conclude that the scattering length is between 1 to 3 nm.The existence of rich varieties of stable isotopes of five bosons (^<168>Yb, ^<170>Yb, ^<172>Yb, ^<174>Yb, and ^<176>Yb) and two fermions (^<171>Yb and ^<173>Yb) will allow us to study various interesting quantum degenerate gases using Yb atoms. In addition, the successful demonstration of Yb BEC in the present work would also promise the formation of BEC of alkali-earth atoms.