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

TL;DR

The paper tackles electrically controllable spin qubits in pristine single-layer graphene by leveraging strain-induced confinement from a nanobubble to form double quantum dots and by introducing tunable Rashba spin-orbit coupling ($\lambda_R$) and Zeeman fields ($\Delta_z$). Using KWANT-based tight-binding quantum transport simulations and a minimal four-band model $H(\delta,\Delta_z,\lambda_R,\varepsilon_0)$, it reveals two detuning-dependent avoided crossings, the spin-conserving gap $\Delta_{sc}$ and the spin-flip gap $\Delta_{sf}$, with $\Delta_{sc}$ decreasing and $\Delta_{sf}$ increasing as $\lambda_R$ grows. Time-domain Lindblad simulations show detuning-dependent Rabi oscillations, distinguishing two regimes: a spin-conserving regime near zero detuning and a spin-flip regime near $\delta=\delta_0$, both tunable by $\lambda_R$ and $\Delta_z$. The results highlight strain-engineered graphene as a viable, scalable platform for spin qubits, combining mechanical control, SOC tunability, and electrostatic detuning within a single device architecture.

Abstract

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.

Figures (4)

• Figure 1: Schematics of the strain-engineered graphene p--n junction spin-qubit device and detuning-induced spin transitions.a Device geometry of the strained graphene nanobubble located at the interface of the $p$--$n$ junction under a perpendicular external magnetic field $\vec{B}$. The localized strain creates a confined potential landscape within the nanobubble region. The blue line shows a quantum Hall current flow. Inset: Enlarged view of the graphene nanobubble illustrating the locally induced strain and curvature. The strain-induced confinement generated by the associated pseudomagnetic field (PMF) is depicted beneath the nanobubble. b Symmetric double-quantum-dot potential and parallel spin configuration in the absence of electrical detuning (spin-conserving transition). c Asymmetric confinement potential induced by detuning, enabling hybridized interdot tunneling accompanied by spin-flip transitions mediated by Rashba spin-orbit coupling (RSOC).
• Figure 2: Quantum conductance including RSOC and the Zeeman field.a Quantum conductance map under the Zeeman field with RSOC. b The zoomed-in views around the single quantum dot (SQD) ground level in the range of $h_{0}=4.1$ to $4.5$ nm. c Quantum conductance line as a function of $y_0$, representing the position of the $p$-$n$ interface channel. d The local density of states (LDOS) and current density plots corresponding to the colored circles in c.
• Figure 3: Detuning-dependent energy spectrum and gap evolution. a Detuning energy spectrum of the double quantum dot (DQD) as a function of the detuning parameter $\delta$ for $\lambda_R$ = 13.6 $meV$ and $\Delta_z$ = 1.7 $meV$. The labels $L$ and $R$ denote the left and right QD states at $K$-valley (positive $y_0$ region), respectively, and the $\uparrow$ and $\downarrow$ indicate the spin states. The gaps $\Delta_{\mathrm{sc}}$ and $\Delta_{\mathrm{sf}}$ correspond to the energy splittings at zero detuning ($\delta=0$) and at finite detuning ($\delta$ = $\delta_0$), respectively. For $\Delta_{\mathrm{sc}}$, $up$ and $lo$ denote the upper and the lower position of energy, respectively. b Evolution of the energy gaps $\Delta$ as a function of the ratio of $\lambda_R$ to $\Delta_z$, obtained from numerical simulations. Here, $\Delta_{\mathrm{sc}}$ is the average value of $\Delta_{\mathrm{sc,up}}$ and $\Delta_{\mathrm{sc,lo}}$. The green area corresponds to the value of RSOC strength (1--17 $meV$) measured in the SLG/TMD heterostructureAvsar2014Gmitra2015Wang2016Wang2015Yang2017Gerber2025Masseroni2024Kurzmann2021Dulisch2025. c Evolution of the energy gaps $\Delta$ as a function of the ratio of $\lambda_R$ to $\Delta_z$, extracted from the analytical four-band Hamiltonian model. Here, $\lambda_{R}$ is taken as 4$n$ times $\Delta_{z}$.
• Figure 4: Rabi oscillation maps as a function of RSOC strength. Rabi oscillation maps are shown for the spin-conserving regime $\delta$ = 0 (a,c,e) and the spin-flip regime $\delta$=$\delta_0$(b,d,f). From left to right, the Rashba coupling strength $\lambda_R$ corresponds to 0, 13, and 24 meV, respectively. Each color map displays the Rabi frequency as a function of the driving detuning energy. The horizontal axis represents the evolution time, and the vertical axis denotes the detuning offset $\Delta \delta$.