  Radio-frequency reflectometry allows for fast and sensitive electrical
readout of charge and spin qubits hosted in quantum dot devices coupled to
resonant circuits. Optimizing readout, however, requires frequency tuning of
the resonators and impedance matching. This is difficult to achieve using
conventional semiconductor or ferroelectric-based varactors in the detection
circuit as their performance degrades significantly in the mK temperature range
relevant for solid-state quantum devices. Here we explore a different type of
material, strontium titanate, a quantum paraelectric with exceptionally large
field-tunable permittivity at low temperatures. Using strontium titanate
varactors we demonstrate perfect impedance matching and resonator frequency
tuning at 6 mK and characterize the varactors at this temperature in terms of
their capacitance tunability, dissipative losses and magnetic field
sensitivity. We show that this allows us to optimize the radio-frequency
readout signal-to-noise ratio of carbon nanotube quantum dot devices to achieve
a charge sensitivity of 4.8 $\mu$e/Hz$^{1/2}$ and capacitance sensitivity of
0.04 aF/Hz$^{1/2}$.
