Gate-Sensing Charge Pockets in the Semiconductor-Qubit Environment

Dispersive gate sensing (DGS) uses radio-frequency (rf) reflectometry to locally probe the quantum capacitance of a gate electrode. Applying DGS to heterostructure-based qubit devices, we report the repeated observation of anomalous signals that we attribute to pockets of charge in the potential landscape likely under, and surrounding, the surface gates that define quantum-dot qubits. Interestingly, these charge pockets appear to evade detection with conventional charge sensors but manifest strongly in the response of the gate sensor. Configuring a quantum point contact (QPC) as a highly localized heat source, we show how these charge pockets likely form close to the end of the gate electrodes, in close proximity to gate-defined qubits. The presence of uncontrolled charge may lead to offsets in gate voltage and further contribute to charge noise that produces decoherence in semiconductor qubits.