  The quantized version of the anomalous Hall effect has been predicted to
occur in magnetic topological insulators, but the experimental realization has
been challenging. Here, we report the observation of the quantum anomalous Hall
(QAH) effect in thin films of Cr-doped (Bi,Sb)2Te3, a magnetic topological
insulator. At zero magnetic field, the gate-tuned anomalous Hall resistance
reaches the predicted quantized value of h/e^2,accompanied by a considerable
drop of the longitudinal resistance. Under a strong magnetic field, the
longitudinal resistance vanishes whereas the Hall resistance remains at the
quantized value. The realization of the QAH effect may lead to the development
of low-power-consumption electronics.
