  We study the single particle scattering process in a coherent multi-site
system consisting of a tight-binding ring threaded by an Aharonov-Bohm flux and
several attaching leads. The asymmetric behavior of scattering matrix is
discovered analytically in the framework of both Bethe Ansatz and Green's
function formalism. It is found that, under certain conditions, a three-site
electronic system can behave analogous to a perfect semiconductor diode where
current flows only in one direction. The general result is also valid for a
neutral particle system since the effective magnetic flux may be implemented by
a globe rotation. This observation means that the three-site system can serve
as an orientation measuring gyroscope due to the approximate linear dependence
of the current difference of two output leads on the rotational angular
velocity.
