  Radio frequency reflectometry techniques enable high bandwidth readout of
semiconductor quantum dots. Careful impedance matching of the resonant circuit
is required to achieve high sensitivity, which however proves challenging at
cryogenic temperatures. Gallium arsenide-based voltage-tunable capacitors,
so-called varactor diodes, can be used for in-situ tuning of the circuit
impedance but deteriorate and fail at temperatures below 10 K and in magnetic
fields. Here, we investigate a varactor based on strontium titanate with
hyperabrupt capacitance-voltage characteristic, that is, a capacitance
tunability similar to the best gallium arsenide-based devices. The varactor
design introduced here is compact, scalable and easy to wirebond with an
accessible capacitance range from 45 pF to 3.2 pF. We tune a resonant
inductor-capacitor circuit to perfect impedance matching and observe robust,
temperature and field independent matching down to 11 mK and up to 2 T in-plane
field. Finally, we perform gate-dispersive charge sensing on a
germanium/silicon core/shell nanowire hole double quantum dot, paving the way
towards gate-based single-shot spin readout. Our results bring small, magnetic
field-resilient, highly tunable varactors to mK temperatures, expanding the
toolbox of cryo-radio frequency applications.
