In-situ charge noise probe through the valley splitting

In semiconductor spin qubits, charge noise leads to fluctuations in the quantum dot confining potential that limit gate fidelity and device scaling. Although charge noise has been characterized through power spectral density measurements, it has not been measured during qubit operation. Therefore, we propose a method for measuring voltage fluctuations in situ by coupling a microwave resonator to the valley degree of freedom of the silicon spin qubit. Interface roughness or impurities in the quantum well induce a dipole moment between the valley states allowing the resonator to couple to the valley states. The valley splitting depends on the electric field and is therefore affected by charge noise. We derive the driving frequency where the valley-resonator system is most sensitive to shifts in the valley splitting. Charge-noise-induced changes in the valley splitting can be detected by continuously measuring the out-of-phase output voltage without collapsing the qubit state.