Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

Abstract

Coherent manipulation of the binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid-state systems, whereas exploitation of the valley has only recently been started, albeit without control at the single-electron level. Here, we show that van der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunnelling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.

Spatial confinement, magnetic localization, and their interactions on massless Dirac fermions

“…For example, the massless Dirac fermions in graphene monolayer can be temporarily trapped to form quasibound states by spatial-varying electrostatic potentials (6,7,11,12) or completely localized into Landau levels (LLs) in large perpendicular magnetic fields (17)(18)(19)(20)(21)(22). By combining two different methods, i.e., the electrostatic potentials and the magnetic fields, in the confinement of the massless Dirac fermions, it is interesting to note that the studied graphene system could exhibit exotic properties that are beyond those already imaged (2,8,9,13,16). For example, the  Berry phase of quasiparticles in a graphene quantum dot can be switched on and off by the magnetic fields (2,13), which cannot be realized by purely using electrostatic potentials or magnetic fields (23)(24)(25).…”

Movable Valley Switch Driven by Berry Phase in Bilayer-Graphene Resonators

“…1(c) and see Fig. S1 for details of analysis [29]), as observed in Bernal bilayer graphene [33,34]. Figure 1 Besides the well-defined Landau levels of massive Dirac fermions and the lowenergy gap, we also observe quadruplet of charging peaks in the tunneling spectra, as shown in Fig.…”

“…Figure 1 Fig. 1(a)) arises from the A/B atoms' asymmetry in the Bernal bilayer graphene, as observed previously [33,34]. To further identify the stacking order of the adjacent bilayer graphene, scanning tunneling spectroscope (STS) measurements in various magnetic fields were carried out, as shown in Fig.…”

CUBIT: Capacitive qUantum BIT

“…1 is routinely used owing to its peculiar electronic and optical properties [12,13]. Quite interestingly, its application to a quantum-dot charge qubit design has been also demonstrated [14], which is however incompatible with the conventional superconducting quantum circuits.…”