25. As shown in the figure, in rhombus $A B C D$, $\angle A B C=120^{\circ}, E$ is the midpoint of side $A B$, $P$ is a moving point on side $A C$, the minimum value of $P B+P E$ is $\sqrt{3}$, then the length of $A B$ is $\qquad$.
answer: 2