Short answer: Magic-angle twisted bilayer graphene can host superconducting regions defined by electrostatic gates, and researchers have used those to build Josephson junctions. A 2021 Nature Nanotechnology paper reports gate-defined junctions and a single-electron transistor in this material, and a 2026 Springer paper proposes theoretically that such a junction could be turned into a protected 0-pi qubit. The qubit is a proposal, and no qubit is reported built in the abstracts.

The 2021 devices

The Nature Nanotechnology abstract describes magic-angle twisted bilayer graphene (MATBG) as a tunable two-dimensional platform with metal, insulator and superconductor states. The authors engineer Josephson junctions and tunnelling transistors defined only by electrostatic gates. The multi-gated geometry gives independent control of the weak link, the barriers and the tunnelling electrodes. They report that the junctions show non-local electrodynamics in a magnetic field, in agreement with Pearl theory for ultrathin superconductors. Using the intrinsic bandgaps, they demonstrate edge tunnelling spectroscopy in the same devices and measure the energy spectrum of MATBG in its superconducting phase. A double-barrier geometry lets a device act as a single-electron transistor with Coulomb blockade. The authors say these devices may find applications in graphene-based tunable superconducting qubits, on-chip superconducting circuits and electromagnetic sensing, which is an outlook.

The Josephson diode effect

The 2026 Springer abstract notes that gate-defined Josephson junctions in MATBG have since been fabricated. A local gate creates two superconducting regions joined by an interaction-driven valley-polarized state as the weak link. Because of spontaneous time-reversal and inversion symmetry breaking, a Josephson diode effect has been observed with no external magnetic field. A separate 2026 Nano Letters article is titled "Gate-Tunable Josephson Diodes in Magic-Angle Twisted Bilayer Graphene"; only its title and author list could be read here, so this post does not describe its findings.

A proposed 0-pi qubit

In the Springer paper, the authors point out that when the nonreciprocity efficiency, which measures the sign and strength of the diode effect, changes sign, the energy-phase relation of the junction is approximately proportional to cos(2 phi). They show theoretically that such a junction, shunted by a capacitor, could realize the 0-pi qubit, which is protected from decoherence caused by local perturbations. They also say that in a regime with large nonreciprocity efficiency, transmon-like qubits with large anharmonicity could be realized. Both claims are results of their analysis.

Limits

The 2021 paper reports devices, not qubits. The 2026 paper is a theoretical proposal, and the abstract describes no fabricated or measured qubit. Whether the protection would hold in a real device, with real disorder and noise, is not answered by either abstract.

FAQ

What is magic-angle graphene?

Two graphene layers twisted by a particular angle, which the 2021 abstract describes as a tunable platform with several electronic phases.

What is a gate-defined Josephson junction?

Two superconducting regions joined by a weak link, all set by voltages on nearby electrodes, per the 2021 abstract.

What is a 0-pi qubit?

A qubit design the 2026 authors say is protected from decoherence caused by local perturbations.

Has a graphene qubit been built this way?

Not according to these abstracts. The 2026 paper is theoretical.

What is the Josephson diode effect?

A direction-dependent supercurrent that the 2026 abstract says has been observed without an external magnetic field.

All field notes