The SMART protocol for scalable quantum computing

Global control strategies for arrays of qubits are a promising pathway to scalable quantum computing. A continuous-wave global field provides decoupling of the qubits from background noise. However, this approach is limited by variability in the parameters of individual qubits in the array. Here we show that by modulating a global field simultaneously applied to the entire array, we are able to encode qubits that are less sensitive to the statistical scatter in qubit resonance frequency and microwave amplitude fluctuations. We name this approach the SMART (Sinusoidally Modulated, Always Rotating and Tailored) qubit protocol. We also report the experimental implementation of the SMART protocol in a single spin confined in a SiMOS quantum dot and confirm the optimal modulation conditions predicted from theory. Universal control of a single qubit is demonstrated using modulated Stark shift control via the local gate electrodes. We measure an extended coherence time of 2 ms and an average Clifford gate fidelity > 99%, constituting a significant improvement over a conventional spin qubit. This work shows that future scalable spin qubit arrays could be operated using global microwave control and local gate addressability, while maintaining robustness to experimental inhomogeneities.