Benchmarking low-power flopping-mode spin-qubit fidelities in Si/Si _0.7 Ge _0.3 devices with alloy disorder

In the ``flopping-mode'' regime of electron spin resonance, a single electron confined in a double quantum dot is electrically driven in the presence of a magnetic-field gradient. The increased dipole moment of the charge in the flopping mode significantly reduces the amount of power required to drive spin rotations; however, the susceptibility of flopping-mode spin qubits to charge noise, and consequently their overall performance, has not been examined in detail. In this work, we simulate single-qubit gate fidelities of electrically driven spin rotations in an ensemble of devices configured to operate in both the single-dot and flopping-mode regimes. Our model accounts for the valley physics of conduction-band electrons in silicon and realistic alloy disorder in the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$ barrier layers, allowing us to investigate device-to-device variability. We include charge and magnetic noise, as well as spin-relaxation processes arising from charge noise and electron-phonon coupling. We find that the two operating modes exhibit significantly different susceptibilities to the various noise sources, with valley splitting and spin-relaxation times also playing a role in their relative performance. For realistic noise strengths, we find that single-dot gate fidelities are limited by magnetic noise, while flopping-mode fidelities are primarily limited by charge noise and spin relaxation. For sufficiently long spin-relaxation times, flopping-mode spin operation is feasible with orders-of-magnitude lower drive power and gate fidelities that are on par with conventional single-dot electric dipole spin resonance.