  We show that Maxwell's demon-like nonreciprocity can be supported in a class
of non-Hermitian gyrotropic metasurfaces in the linear regime. The proposed
metasurface functions as a transmission-only Maxwell's demon operating at a
pair of photon energies. Based on multiple scattering theory, we construct a
dual-dipole model to explain the underlying mechanism that leads to the
antisymmetric nonreciprocal transmission. The results may inspire new designs
of compact nonreciprocal devices for photonics.
