Quantum paraelectric varactors for radio-frequency measurements at mK temperatures

TL;DR

This paper demonstrates the use of strontium titanate quantum paraelectric varactors for efficient radio-frequency readout of quantum dot devices at millikelvin temperatures, enabling improved impedance matching and tunability.

Contribution

It introduces and characterizes strontium titanate varactors for RF tuning at mK temperatures, showing their advantages over conventional varactors in quantum device readout.

Findings

Achieved perfect impedance matching at 6 mK using strontium titanate varactors.

Demonstrated high capacitance tunability and low losses at millikelvin temperatures.

Enhanced charge and capacitance sensitivity in quantum dot RF readout.

Abstract

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$^{1/2}$.