  In a magnetic topological insulator, nontrivial band topology conspires with
magnetic order to produce exotic states of matter that are best exemplified by
quantum anomalous Hall (QAH) insulators and axion insulators. Up till now, such
magnetic topological insulators are obtained by doping topological insulators
with magnetic atoms. The random magnetic dopants, however, inevitably introduce
disorders that hinder further exploration of quantum effects in the material.
Here, we resolve this dilemma by probing quantum transport in MnBi$_2$Te$_4$
thin flake - a topological insulator with intrinsic magnetic order. In this
layered van der Waals crystal, the ferromagnetic layers couple anti-parallel to
each other, so MnBi$_2$Te$_4$ is an antiferromagnet. A magnetic field, however,
aligns all the layers and induces an interlayer ferromagnetic order; we show
that a quantized anomalous Hall response emerges in atomically thin
MnBi$_2$Te$_4$ under a moderate magnetic field. MnBi$_2$Te$_4$ therefore
becomes the first intrinsic magnetic topological insulator exhibiting quantized
anomalous Hall effect. The result establishes MnBi$_2$Te$_4$ as an ideal arena
for further exploring various topological phenomena.
