Optimized Backgate Design for Enhancing Coherence Times in Graphene Gatemons*

Gate-tunable Josephson junction-based qubits (gatemons) have emerged as a promising approach to mitigate some of the key challenges in standard, flux-tunable transmon superconducting qubits, such as qubit sensitivity to flux noise, crosstalk, and power dissipation. Unlike SQUIDs, gatemons do not rely on flux biases and can be controlled via an electrostatic gate. While gatemons have been realized in various systems, including semiconducting nanowires [1], two-dimensional electron gases [2], and graphene [3], their coherence times remain a limiting factor for achieving high-fidelity multi-qubit operations. Here, we present a graphene-based differential (floating) qubit design in which two capacitor pads are symmetrically coupled to the back gate line with equal strength. This geometry is expected to significantly inhibit energy decay through this channel, which is suspected to be the primary source of dissipation limiting prior work [3]. We explore these improvements in the context of device design, fabrication, and microwave measurements.

[1] Larsen et al. PRL (2015)

[2] Casparis et al. Nat. Nanotech. (2018)

[3] Wang*, Rodan-Legrain* et al. Nat. Nanotech. (2019)