Fast charge noise sensing using a spectator valley state in asinglet-triplet qubit

Semiconductor spin qubits are a promising platform for quantum computing but remain vulnerable tocharge noise. Accurate, in situ measurement of charge noise could enable closed-loop control andimprove qubit performance. Here, we propose a method for real-time detection of charge noise usinga silicon singlet-triplet qubit in which one electron is initialized in an excited valley state. This valleyexcitation acts as a spectator degree of freedom coupled to a high-quality resonator via the exchangeinteraction, which is sensitive to charge-noise-induced voltage fluctuations. Dispersive readout of theresonator enables a continuous, classical measurement of exchange fluctuations during qubitoperation. Signal-to-noise analysis shows that, under realistic device parameters, sub-millisecondmeasurement times are achievable using a quantum-limited amplifier. Even without such an amplifier,similar performance is achievable with appropriately engineered resonator parameters. This approachenables real-time monitoring of slow drifts in exchange, opening the door to feedback and feedforwardstrategies for maintaining high-fidelity quantum operations. Importantly, the protocol preserves spincoherence and can be executed concurrently with qubit logic gates.