Quantum coherence and control is foundational to the science and engineering of quantum systems 1,2 . In van der Waals (vdW) materials, the collective coherent behavior 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 traveling ballistically 4,5 . In addition to the potential for advancing extensible quantum computing technology, our results represent a new approach to studying vdW materials using microwave photons in coherent quantum circuits.
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