Electrically tunable spin qubits in strain-engineered graphene p-n junctions

Strain engineering enables quantum confinement in pristine graphene without degrading its intrinsic mobility and spin coherence. Here, we extend previously proposed strain-induced charge-qubit architectures by incorporating spin degrees of freedom through Rashba spin-orbit coupling (RSOC) and Zeeman fields, enabling spin-qubit operation in single-layer graphene (SLG). In a graphene p-n junction, a strain-induced nanobubble generates a pseudo-magnetic field that forms double quantum dots with gate-tunable level hybridization. Tight-binding quantum transport simulations and a four-band model reveal two distinct avoided crossings: spin-conserving gaps at zero detuning and spin-flip gaps at finite detuning, the latter increasing with SOC strength while the former decreases. Time-domain simulations confirm detuning-dependent Rabi oscillations corresponding to these two operational regimes. These results demonstrate that strain-induced confinement combined with tunable SOC provides a viable mechanism for coherent spin manipulation in pristine graphene, positioning strained SLG as a promising platform for scalable spin-based quantum technologies.

💡 Research Summary

The paper presents a comprehensive theoretical proposal for realizing electrically tunable spin qubits in single‑layer graphene (SLG) by exploiting strain‑induced pseudo‑magnetic fields (PMFs) in a p‑n junction geometry. A localized nanobubble at the p‑n interface creates a highly non‑uniform strain field, which generates a strong PMF that confines Dirac electrons into discrete Landau‑like levels. This confinement naturally yields a double‑quantum‑dot (DQD) potential landscape without the need for electrostatic gates that would otherwise be ineffective in pristine graphene.

To endow the system with spin functionality, the authors incorporate Rashba spin‑orbit coupling (RSOC) and an external Zeeman field (ΔZ). RSOC can be tuned by a perpendicular electric field or by proximity to transition‑metal‑dichalcogenide (TMD) layers such as WSe₂, while ΔZ is controlled by a magnetic field. The combined effect of PMF, RSOC, and Zeeman splitting produces two distinct avoided‑crossing regimes as a function of the inter‑dot detuning parameter δ, which is experimentally set by a gate voltage that makes the p‑n junction asymmetric.

In the symmetric case (δ = 0) the avoided crossing occurs between states of the same spin orientation (|L↑⟩↔|R↑⟩ and |L↓⟩↔|R↓⟩), generating a spin‑conserving gap Δ_sc. This gap diminishes as RSOC strength λ_R increases because RSOC mixes spin channels but does not directly open the gap. In the asymmetric case (finite δ) the avoided crossing involves opposite‑spin states (|L↑⟩↔|R↓⟩ etc.), creating a spin‑flip gap Δ_sf that grows linearly with λ_R. Tight‑binding quantum transport simulations confirm these trends: conductance maps show asymmetric resonance lines when RSOC is present, and local density of states (LDOS) visualizations reveal clear single‑dot and double‑dot regimes.

An effective four‑band Hamiltonian H(δ,ΔZ,λ_R,ε_0) = −αδ τ_z⊗σ_0 + ε_0 e^{−ζλ_R} τ_x⊗σ_0 + ΔZ σ_z + γλ_R^2 τ_0⊗σ_z + βλ_R^2 τ_x⊗σ_y captures the essential physics. Here τ_i act on the dot subspace and σ_i on real spin. Fitting parameters α, β, γ, ζ reproduce the tight‑binding spectra and allow systematic exploration of how Δ_sc and Δ_sf depend on detuning, RSOC, Zeeman splitting, and inter‑dot coupling.

The dynamical properties are investigated using the Lindblad master equation to model driven two‑level dynamics with decoherence. Rabi oscillations are simulated for both avoided‑crossing regimes. In the spin‑conserving case the Rabi frequency is modest and scales weakly with λ_R, whereas in the spin‑flip case the frequency is significantly higher and directly proportional to λ_R. By adjusting δ, λ_R, and ΔZ, the qubit can be switched between charge‑qubit‑like operation (spin‑conserving) and genuine spin‑qubit operation (spin‑flip), offering a versatile platform where mechanical strain, electric fields, and magnetic fields serve as independent control knobs.

Overall, the work demonstrates that (i) strain‑engineered PMFs provide high‑quality quantum confinement in pristine graphene, (ii) tunable RSOC and Zeeman fields enable coherent spin manipulation without sacrificing graphene’s intrinsic high mobility and long spin coherence times, and (iii) gate‑controlled detuning allows electrical selection of the qubit modality. These findings position strained SLG as a promising, scalable candidate for spin‑based quantum information processing, bridging the gap between charge‑only graphene qubits and more conventional semiconductor spin qubits while preserving the unique advantages of two‑dimensional Dirac materials.