  The few-layer graphene quantum dot provides a promising platform for quantum
computing with both spin and valley degrees of freedom. Gate-defined quantum
dots in particular can avoid noise from edge disorders. In connection with the
recent experimental efforts [Y. Song et al., Nano Lett. 16, 6245 (2016)], we
investigate the bound state properties of trilayer graphene (TLG) quantum dots
(QDs) through numerical simulations. We show that the valley degeneracy can be
lifted by breaking the time reversal symmetry through the application of a
perpendicular magnetic field. The spectrum under such a potential exhibits a
transition from one group of Landau levels to the other group, which can be
understood analytically through perturbation theory. Our results provide
insight to the transport property of TLG QDs, with possible applications to
study of spin qubits and valleytronics in TLG QDs.
