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arXiv:2605.24790

High-fidelity EDSR in Si/SiGe Wiggle Wells

Hudaiba Soomro, Minyoung Kim, Avani Vivrekar, M. A. Eriksson, Benjamin D. Woods, Mark Friesen

cond-mat.mes-hallquant-phSubmitted May 24, 2026• Announced May 26, 2026• Revised Jun 1, 2026 (v2)
v1 · May 24, 2026v2 · Jun 1, 2026
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Abstract

Si/SiGe quantum wells that incorporate Ge concentration oscillations, known as long-period Wiggle Wells, have been shown to enhance the Dresselhaus spin-orbit coupling of conduction-band electrons. Such intrinsic spin-orbit coupling is desirable when performing spin-qubit gate operations based on electric dipole spin resonance (EDSR) because it eliminates the need for external micromagnets. However, random-alloy disorder plays a key role in this materials system by spatially randomizing the valley splitting and the valley phase $φ_{s,s}$, and it has not been fully accounted for in recent EDSR analyses. Here, we show that alloy disorder affects EDSR in two main ways. First, the Rabi frequency $Ω$ acquires a dependence on the valley phase, given by $\cosφ_{s,s}$, which causes spatial randomization of $Ω$. Despite this variability, we show that fast EDSR can be achieved at most locations across a given sample. Second, a new Rabi driving mechanism emerges, enabled by valley dipoles and the hybridization of ground and excited valley states, which arise from alloy disorder and EDSR driving, respectively. This mechanism is dominant in regions of low valley splitting. Alloy disorder can therefore strengthen EDSR, but it can also cause gradients in $Ω$ that lead to dephasing in the rotating frame. We explore this problem by first locating "sweet spots," where EDSR is relatively insensitive to electric-field fluctuations. We then show that high-fidelity Rabi oscillations can be achieved in the presence of realistic charge noise. These results suggest that Wiggle Wells are a promising platform for high-quality, micromagnet-free gate operations.

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18 pages

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Si/SiGe quantum wells that incorporate Ge concentration oscillations, known as long-period Wiggle Wells, have been shown to enhance the Dresselhaus spin-orbit coupling of conduction-band electrons. Such intrinsic spin-orbit coupling is desirable when performing spin-qubit gate operations based on electric dipole spin resonance (EDSR) because it eliminates the need for external micromagnets. However, random-alloy disorder plays a key role in this materials system by spatially randomizing the valley splitting and the valley phase φs,sφ_{s,s}φs,s​, and it has not been fully accounted for in recent EDSR analyses. Here, we show that alloy disorder affects EDSR in two main ways. First, the Rabi frequency ΩΩΩ acquires a dependence on the valley phase, given by cos⁡φs,s\cosφ_{s,s}cosφs,s​, which causes spatial randomization of ΩΩΩ. Despite this variability, we show that fast EDSR can be achieved at most locations across a given sample. Second, a new Rabi driving mechanism emerges, enabled by valley dipoles and the hybridization of ground and excited valley states, which arise from alloy disorder and EDSR driving, respectively. This mechanism is dominant in regions of low valley splitting. Alloy disorder can therefore strengthen EDSR, but it can also cause gradients in ΩΩΩ that lead to dephasing in the rotating frame. We explore this problem by first locating "sweet spots," where EDSR is relatively insensitive to electric-field fluctuations. We then show that high-fidelity Rabi oscillations can be achieved in the presence of realistic charge noise. These results suggest that Wiggle Wells are a promising platform for high-quality, micromagnet-free gate operations.
Hole spin qubits in semiconductor quantum dots (QDs) enable high-fidelity all-electric control, but conventional electric dipole spin resonance (EDSR) can require substantial rf drive power at the low magnetic fields that are favorable for qubit coherence and readout. In planar Ge hole spin qubits, this can reach -27 dBm at the device, posing challenges for scalable architectures due to heating and crosstalk. Here, we demonstrate a flopping-mode (FM) qubit in Ge, where a single spin is delocalized in a double QD, combining first-order protection against charge noise with exceptionally efficient electric driving. By mapping out coherence sweet-spots as a function of magnetic field orientation we achieve T2∗=1.4μsT_2^*= 1.4μ\mathrm{s}T2∗​=1.4μs, T2Hahn=11.5μsT_2^{\mathrm{Hahn}}= 11.5 μ\mathrm{s}T2Hahn​=11.5μs, T2φ,CPMG32=130μsT^{φ, \mathrm{CPMG32}}_2= 130 μ\mathrm{s}T2φ,CPMG32​=130μs, and T1=226μsT_1= 226 μ\mathrm{s}T1​=226μs, and a single-qubit gate fidelity of up to 99.76%\%% for a gate time tXπ=88t_{Xπ} = 88tXπ​=88 ns. Importantly, these results are obtained at a nearly in-plane magnetic field of 5 mT using only -52 dBm drive power at the device. We further find that qubit relaxation in this regime is consistent with a two-photon Orbach process, providing a route for further optimization. Our results demonstrate that FM-EDSR supports ultra-low-power, high-fidelity single-qubit operations, improvements that could benefit scalable hole-spin-based architectures and hybrid spin-photon interfaces.
There have been impressive breakthroughs in semiconductor quantum dots in recent years, with single- and two-qubit gate fidelities matching other leading platforms and scalability still remaining a relative strength; however, due to qubit wiring considerations, mobile electron architectures have been proposed to facilitate upward scaling, and this opens the possibility of enlarging the motional amplitude for electric-dipole spin resonance (EDSR)-based qubit manipulation. In this work, we examine and demonstrate the possibility of significantly outperforming static-EDSR-type single-qubit pulsing by taking advantage of greater spatial mobility to achieve higher Rabi frequencies and reduce the effect of charge noise. Our theoretical results indicate that fidelities are ultimately bottlenecked by spin-valley physics, which can be suppressed through the use of quantum optimal control. We demonstrate that, across different potential regimes and competing physical models, shuttling-based single-qubit gates retain significant advantages over existing alternatives.
Small and device-dependent valley splittings remain a key challenge for electron spin qubits in silicon (Si), directly limiting qubit fidelity, device uniformity, and the scalability of Si-based quantum processors. In silicon-germanium (SiGe) heterostructures, this problem can be addressed through engineering of the epitaxial layer stack. Several heuristic strategies have been proposed to enhance the energy gap between the two nearly degenerate valley states in strained Si/SiGe quantum wells (QWs), e.g., sharp Si/SiGe interfaces, Ge spikes, or oscillating Ge concentrations within the QW. Here, we develop a systematic variational optimization approach to compute optimal Ge concentration profiles that enhance selected properties of the intervalley coupling matrix element. Our free-shape optimization framework is augmented by realistic technological constraints to ensure feasibility of the resulting epitaxial profiles and is based on an effective-mass envelope-function theory accounting for strain and compositional alloy disorder. Previously proposed heterostructure designs are recovered as special cases of the constrained optimization problem. Our main result is a novel heterostructure design, which we refer to as the "modulated wiggle well", providing both a large deterministic enhancement of the valley splitting and a strong suppression of disorder-induced variability. In addition, this design enables wide electrical tunability of the valley splitting - from approximately 200 μeV200\,μ\text{eV}200μeV to above 1 meV1\,\text{meV}1meV - offering new opportunities for engineering robust and switchable silicon qubits.
While bulk silicon has long been understood to exhibit relatively weak spin-orbit coupling (SOC), confinement of electrons to quantum dots (QDs) at a silicon heterointerface results in significantly larger SOC. This is a concern for electron spin qubit performance, as intravalley and intervalley SOC can significantly perturb the operation of electron spin qubits. While these interactions can be harnessed to drive coherent rotations in a singlet-triplet qubit, coupling to low-lying excited valley states can lead to undesirable spin relaxation when valley splitting is on resonance with the Zeeman energy. In this work, we measure the angular dependence of the interfacial spin-orbit interaction as a function of the direction and magnitude of an applied external magnetic field in SiMOS and Si/SiGe heterostructures, two common material platforms for silicon spin qubits. We construct a physical model that accurately infers intra- and inter-valley SOC physics from fits to the data, allowing for a direct comparison between these two material systems. For the devices measured we find that, while the ggg-factor differences are comparable, the SiMOS QDs exhibit an order of magnitude larger spin-valley coupling than for Si/SiGe. Moreover, we find that the angular dependence of the spin-valley coupling is similar for both devices, with similar magnetic field orientations minimizing the spin-valley coupling. Our work points towards operational schemes for optimizing spin-valley coupling to avoid or exploit this mechanism for qubit operation.
We develop a symmetry-based theory of spin-orbit-valley coupling in silicon that reveals an intrinsic source of Dresselhaus spin-orbit coupling independent of interfaces or external electric fields. Treating the valley degree of freedom as a symmetry-carrying quantum degree of freedom, we show that the Dresselhaus interaction is necessarily valley off-diagonal and that a bulk contribution proportional to the valley Pauli matrix τ1τ_1τ1​ is symmetry allowed. Tight-binding calculations yield a bulk coupling more than an order of magnitude larger than typical interface-induced spin-orbit coupling; achieving the same energy scale through the interface-induced mechanism would require electric fields roughly 50 times larger than typical fields. We further derive the symmetry-allowed spin-valley couplings generated by magnetic-field gradients and show how they account for the valley-dependent Zeeman splitting observed in micromagnet experiments. A slight tilt of the background magnetic field out of the plane produces an additional isotropic contribution linear in BzB_zBz​, providing an experimentally accessible signature of the corresponding coupling constant. Finally, we predict a spin-independent micromagnet-induced valley splitting in the τ3τ_3τ3​ channel, which is distinct from the τ1,2τ_{1,2}τ1,2​ channels generated by alloy disorder and therefore remains robust against disorder-induced cancellation. These results establish valley symmetry as a fundamental ingredient in the spin-orbit physics of silicon and provide new mechanisms for controlling and probing spin and valley degrees of freedom in silicon quantum devices.
Single electron spins can be used to encode and process information in semiconductor quantum devices. Progress has been hindered by materials challenges, such as the small energy splitting between low-lying valley states and hyperfine coupling to nuclear spins. Here we use density functional theory to optimize the valley splitting and spin dephasing time in realistic Si/SiGe heterostructures. Reductions in the Si quantum well width generally increase the valley splitting. However, in narrow quantum wells, a larger fraction of the electronic wavefunction resides in the SiGe buffer layers, which increases the hyperfine coupling with spinful 73^{73}73Ge. Our work shows that Si/SiGe heterostructures with 3~--~4~nm wide quantum wells and 73^{73}73Ge and 29^{29}29Si concentrations of 50 ppm should support average valley splittings EvE_{v}Ev​~>>>~500~μμμeV and spin dephasing times T2∗T_2^*T2∗​ exceeding 15~μμμs assuming an effective quantum dot area of 700 nm2^22. In addition, sharper Si/SiGe interfaces in general result in larger valley splittings and longer spin dephasing times.