Bilayer Graphene Quantum Dots as Spin and Valley Qubit Hosts

Short answer: Bilayer graphene quantum dots are being studied as hosts for spin and valley qubits. A 2023 Nature Physics paper reported valley triplet-singlet relaxation times above 500 ms, and a 2025 Nature Nanotechnology paper reported a spin-valley relaxation time of 38 s at 30 mK. Relaxation time is not coherence time, and manipulation of these states is still ahead.

What makes bilayer graphene different?

The 2023 abstract describes bilayer graphene as a promising platform for electrically controllable qubits in a two-dimensional material. The interest comes from the valley degree of freedom, a two-fold orbital degeneracy arising from the symmetry of the hexagonal crystal structure. The authors argue valleys could be advantageous because known spin- and orbital-mixing mechanisms are unlikely to act on them, and Berry curvature allows electrical control of valley energies. They also say the relaxation time of valley states, which ultimately limits coherence, was not yet known before their work.

What did the 2018 and 2023 papers measure?

A 2018 paper in Physical Review X measured electronic transport through a gate-defined bilayer graphene quantum dot confined by gapped regions and connected to leads via p-n junctions. The authors report single electron and hole occupancy, with charge carriers filled successively up to n equal to 50 and charging energies exceeding 10 meV. For the lowest states they observe valley and Zeeman splittings with a spin g-factor of about 2.

The 2023 Nature Physics paper measured relaxation times of spin and valley states in gate-defined bilayer graphene quantum dots. The authors report that different valley states can be distinguished with a fidelity of over 99%, and that the relaxation time between valley triplets and singlets exceeds 500 ms, more than one order of magnitude longer than for spin states.

What did the 2025 paper add?

The 2025 Nature Nanotechnology paper focuses on the single-carrier quantum dot ground state, which is twofold degenerate with a Kramers pair of opposite spin and valley quantum numbers. The authors explain that an out-of-plane magnetic field breaks time-reversal symmetry through the valley-dependent Berry curvature, so a qubit can be encoded in the spin-valley subspace. They report a spin-valley relaxation time of 38 s at 30 mK, two orders of magnitude longer than the 0.4 s measured for purely spin-blocked states. They also report that the intrinsic Kane-Mele spin-orbit splitting enables single-shot readout of the Kramers doublet even at zero magnetic field, with a fidelity above 99%.

What is still open?

The 2025 authors state a condition: if these long-lived Kramers states also possess long coherence times and can be effectively manipulated, electrostatically defined quantum dots in bilayer graphene may serve as long-lived semiconductor qubits. The 2023 authors likewise describe their work as facilitating future measurements of valley-qubit coherence. So the reported results concern relaxation and readout in individual laboratory devices. This post is based on the abstracts and visible summary text and has not reviewed full texts, and it makes no claim about products or deployments.

FAQ

What is a valley qubit?

In the 2023 abstract, a valley is a two-fold orbital degeneracy that arises from the symmetry of the hexagonal crystal structure of bilayer graphene. Quantum information can be encoded in it.

How long did the states relax in the 2025 experiment?

The authors report a spin-valley relaxation time of 38 s at 30 mK, compared with 0.4 s for purely spin-blocked states.

Does relaxation time equal coherence time?

No. The 2023 abstract says relaxation time ultimately limits coherence, and that valley-qubit coherence measurements are future work.

Has single-shot readout been shown?

The 2025 authors report single-shot readout of the Kramers doublet at zero magnetic field with a fidelity above 99%.