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.

二元自由度的相干操纵是现代量子技术的核心。石墨烯提供两种二元度：电子自旋和谷值。在许多固态系统中已经证明了有效的自旋控制，而对谷的开发直到最近才开始，尽管没有在单电子水平上进行控制。在这里，我们表明范德华斯将石墨烯堆叠到六方氮化硼上提供了自然的谷底控制平台。我们使用由扫描隧道显微镜的尖端诱导的石墨烯量子点，并演示了通过量子点位置的低于10nm的位移可将谷值分裂从-5调整到+10 meV（包括谷值反转）的方法。这将可控谷值分割的范围提高了大约一个数量级。