Gate-tunable Transmon Qubit made with Graphene/hBN Heterostructures*

Graphene – an atomic layer of carbon atoms arranged in a honeycomb lattice – can acquire superconductivity via the proximity effect. We present the fabrication and characterization of a transmon qubit using graphene as a voltage-tunable weak link. The graphene monolayer is encapsulated in hBN crystals from both sides and couples to aluminum electrodes via etched 1-D edges to ensure highly transparent contacts. The critical supercurrent of the S-G-S junction is tuned by an underlying back-gate fabricated on the qubit chip. Using standard dispersive readout techniques, we observe that the frequency and coherence of our qubit exhibit an evident dependence on the Fermi energy – controlled by the back-gate – as expected for graphene weak links. Our result opens a new avenue toward voltage-controlled superconducting qubits, with relevance to certain scalable quantum computing concepts, as well as a novel platform to study mesoscopic physics using circuit QED techniques.