Spin correlation in top-quark production at hadron colliders

We propose techniques to observe the correlation of the spins of top quarks and antiquarks at the Tevatron and the LHC. Observation of the spin correlation would confirm that the top quark decays before its spin flips, and would place a lower bound on the top-quark width and Vtb. The spin correlation may also be a useful tool to study the weak decay amplitude of the top quark.