Real-time charge noise sensing using a spectator valley state

Fast, in situ measurement of charge noise in Si spin qubits could enable closed-loop control and improve qubit performance. We theoretically propose a method to measure charge noise in situ. Our method does not require ancillary spectator qubits but makes use of the valley degree of freedom. Dispersive readout of a high-impedance resonator coupled to virtual valley excitations enables a continuous, classical measurement of local charge-noise-induced voltage fluctuations via the homodyne current during qubit operation. Signal-to-noise analysis shows that, under realistic device parameters, sub-millisecond measurement times are possible using a quantum-limited amplifier. This approach allows the probe to, for example, monitor slow drift in exchange in real time, opening the door to feedback and feedforward strategies for maintaining high-fidelity quantum operations. Importantly, the protocol preserves spin coherence and can be run concurrently with qubit logic gates.