  We propose an intrinsic 3D Fabry-P\'erot type interferometer, coined
"higher-order interferometer", that utilizes the chiral hinge states of
second-order topological insulators and cannot be equivalently mapped to 2D
space because of higher-order topology. Quantum interference patterns in the
two-terminal conductance of this interferometer are controllable not only by
tuning the strength but also, particularly, by rotating the direction of the
magnetic field applied perpendicularly to the transport direction. Remarkably,
the conductance exhibits a characteristic beating pattern with multiple
frequencies with respect to field strength or direction. Our novel
interferometer provides feasible and robust magneto-transport signatures to
probe the particular hinge states of higher-order topological insulators.
