  A p-n junction, an interface between two regions of a material populated with
carriers of opposite charge, is a basic building block of solid state
electronic devices. From the fundamental physics perspective, it often serves
as a tool to reveal the unconventional transport behavior of novel materials.
In this work, we show that a p-n junction made from a three dimensional
topological insulator (3DTI) in a magnetic field realizes an electronic
Mach-Zehnder interferometer with virtually perfect visibility. This is owed to
the confinement of the topological Dirac fermion state to a closed
two-dimensional surface, which offers the unprecedented possibility of
utilizing external fields to design networks of chiral modes wrapping around
the bulk in closed trajectories, without the need of complex constrictions or
etching. Remarkably, this junction also acts as a spin filter, where the path
of the particle is tied to the direction of spin propagation. It therefore
constitutes a novel and highly tunable spintronic device where spin polarized
input and output currents are naturally formed and could be accessed and
manipulated seperately.
