  Traditional polarizer provides a way to convert an unknown polarization into
a specified polarization. According to Malus, the intensity of the transmission
is directly proportional to the square of the cosine of angle between the
transmission axes of the polarizer and the incident polarization. There, the
intensity refers to the collective behavior of many photons. Here we propose a
novel approach to realizing polarization-filtering at single-photon level. We
discuss how a single planepolarized photon transports through a polarized
analyzer generated by a single atom (the so-called "single-atom polarizer"),
and provide a single quantum version of Malus's law. We investigate the quantum
scattering of a single photon by a controllable four-level atom inside a
one-dimensional waveguide. By using real-space theoretical approach, we obtain
analytic expressions of the transmission spectrum of the photon. Then, our
numerical experiments show that the transmitted probability of the incident
photon can be controlled by selecting the polarized-dependent transition
configuration of the driven atom. The application of such a single-atom
polarizer to linear optical quantum information processing is also discussed.
