  We study the two-dimensional motion of a magnetic skyrmion driven by a
ratchetlike polarized electric current that is periodic in both space and time.
Some general cases are considered, in each of which,in the low temperature and
adiabatic limit, regardless of the details of the driving current, the time and
statistical average velocity along any direction is topologically quantized as
a Chern number, multiplied by a basic unit. We make two approaches, one based
on identifying the drift direction, and the other based on the nonhermitian
adiabatic perturbation theory developed for the Fokker-Planck operator. Both
approach applies in the case of periodicity along the direction of the driving
current and homogeneity in the transverse direction, for which the analytical
result is confirmed by our numerical simulation on the constituent spins,and a
convenient experiment is proposed.
