  The discovery of quantum spin Hall materials with huge bulk gaps in
experiment, such as bismuthene, provides a versatile platform for topological
devices. We propose a topological quantum dot (QD) device in bismuthene ribbon
in which two planar magnetization areas separate the sample into a QD and two
leads. At zero temperature, peaks of differential conductance emerge,
demonstrating the discrete energy levels from the confined topological edge
states. The key parameters of the QD, the tunneling coupling strength with the
leads and the discrete energy levels, can be controlled by the planar
magnetization and the sample size. Specially, different from the conventional
QD, we find that the angle between two planar magnetization orientations
provides an effective way to manipulate the discrete energy levels. Combining
the numerical calculation and the theoretical analysis, we identify that such
manipulation originates from the unique quantum confinement effect of the
topological edge states. Based on such mechanism, we find the spin transport
properties of QDs can also be controlled.
