Sub-Kelvin bias and gain circuits for improving charge sensing signal-to-noise in quantum dot qubits

Performance measurements for simple bias and current amplification circuits based on inexpensive, commercial (as well as some custom packaged discrete components) measured at <1 Kelvin are presented and discussed as a strategy for improving bandwidth and reducing noise in charge sensor current signals. Capacitive charge sensing using single electron transistors (SETs) relies on discrimination between high and low current levels that may differ by <10 pA. These currents are commonly sourced and measured by electronics located at room temperature outside of the cryogenic environment, resulting in current loops as large as 10 m in length. Our work seeks to both source and amplify the charge sensing current within the 4 K area of the cryogenic system, thus dramatically reducing the opportunity for parasitic noise source to degrade the charge sensing signal. SET measurements taken using resistive voltage dividers and single transistor gain stages located close to SETs will be presented and discussed.