Quantum coherence and control is foundational to the science and engineering of quantum systems(1,2). In van der Waals materials, the collective coherent behaviour of carriers has been probed successfully by transport measurements(3-6). However, temporal coherence and control, as exemplified by manipulating a single quantum degree of freedom, remains to be verified. Here we demonstrate such coherence and control of a superconducting circuit incorporating graphene-based Josephson junctions. Furthermore, we show that this device can be operated as a voltage-tunable transmon qubit(7-9), whose spectrum reflects the electronic properties of massless Dirac fermions travelling ballistically(4,5). In addition to the potential for advancing extensible quantum computing technology, our results represent a new approach to studying van der Waals materials using microwave photons in coherent quantum circuits.