  Graphene quantum dots provide promising platforms for hosting spin, valley,
or spin-valley qubits. Taking advantage of the electrically generated band gap
and the ambipolar nature, high-quality quantum dots can be defined in bilayer
graphene using natural p-n junctions as tunnel barriers. In these devices,
demonstrating the electrical tunability of the p-n junction barriers and
understanding its physical mechanism, especially in the few-electron regime,
are essential for further manipulating electron's quantum degrees of freedom to
encode qubits. Here, we show the electrostatic confinement of single quantum
dots in bilayer graphene using natural p-n junctions. When the device is
operated in the few-electron regime, the electron tunneling rate is found to be
monotonically tuned by varying gate voltages, which can be well understood from
the view of manipulating the p-n junction barriers. Our results provide an
insightful understanding of electrostatic confinement using natural p-n
junctions in bilayer graphene, which is beneficial for realizing graphene-based
qubits.
