All-optical realization of quantum degeneracy of Yb atoms

镱（Yb）和碱土金属等双电子原子的BEC具有独特的特性，如极窄的组合跃迁，这是未来光频标的理想选择;对外磁场不敏感，这对精密相干原子光学至关重要。本文报道了在一种新型的交叉远离共振阱（FORT）中用全光方法实现Yb原子的BEC。蒸发冷却是通过在2秒内逐渐降低水平FORT束功率来进行的，同时保持垂直FORT波束功率恒定。当最终陷阱深度为2.2，K时，出现了释放后的各向异性膨胀。凝聚体中的原子数通常为5×10^3。临界温度约为0.7，K，峰值原子密度为7×10 ~（13）<14>cm ~ 2<-3>.凝聚体的寿命约为500 ms，我们推断三体损耗系数的上限为4×10^<-30>cm^6 s^<-1>。从凝聚体的行为，我们估计了^ Yb的散射长度<174>。通过与10个TOF时间的水平扩展宽度的拟合获得了0.55，K的化学势，这给出了6 nm的散射长度，不确定度为-5和+10nm。结合我们最近的光缔合实验，我们得出散射长度在1 ~ 3 nm之间的结论。Yb原子中存在着丰富的五种玻色子（~ <168>Yb，~ <170>Yb，~ Yb<172>，~ Yb，<174>~ <176>Yb）和两种费米子（~ <171>Yb，~ <173>Yb）的稳定同位素，这将使我们能够用Yb原子研究各种有趣的量子简并气体。此外，Yb BEC在本工作中的成功演示，也有望形成碱土原子的BEC。

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.