11.如图,正四棱柱ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,AA1=2AB=2AA_{1}=2AB=2AA1=2AB=2,点EEE和FFF分别是线段AC1AC_{1}AC1与BDBDBD上的动点,则EFEFEF间最小距离为((( )))