Superfluidity of Bose—Einstein condensates in ultracold atomic gases*
Zhu Qi-Zhonga), Wu Biaoa),b),c)†
       
(a) A superfluid moves inside a stationary tube. (b) The superfluid is dragged by a tube moving at the speed of v . (c) An impurity moves at v in the superfluid. The two-way arrow indicates the equivalence between different scenarios.