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^3,4,5,6. 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^7,8,9, 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.

量子相干和控制是量子系统^1,2的科学和工程基础。在范德华斯材料中，已经通过传输测量^{3、4、5、6}成功地探测了载体的集体相干行为。但是，时间相干性和控制性，如通过操纵单个量子自由度所举例说明的，尚待验证。在这里，我们演示了结合基于石墨烯的约瑟夫森结的超导电路的这种相干性和控制。此外，我们证明了该装置可以作为电压可调的跨极量子比特^7,8,9运行，其频谱反映了无质量狄拉克费米子的弹道运动^4,5的电子性质。。除了发展可扩展量子计算技术的潜力外，我们的研究结果还提供了一种在相干量子电路中使用微波光子研究范德华材料的新方法。