Radiofrequency reflectometry can provide fast and sensitive electrical read-out of charge and spin qubits in quantum dot devices coupled to resonant circuits. In situ frequency tuning and impedance matching of the resonator circuit using voltage-tunable capacitors (varactors) is needed to optimize read-out sensitivity, but the performance of conventional semiconductor- and ferroelectric-based varactors degrades substantially in the millikelvin temperature range relevant for solid-state quantum devices. Here we show that strontium titanate and potassium tantalate, materials which can exhibit quantum paraelectric behaviour with large field-tunable permittivity at low temperatures, can be used to make varactors with perfect impedance matching and resonator frequency tuning at 6 mK. We characterize the varactors at 6 mK in terms of their capacitance tunability, dissipative losses and magnetic field insensitivity. We use the quantum paraelectric varactors to optimize the radiofrequency read-out of carbon nanotube quantum dot devices, achieving a charge sensitivity of 4.8 μe Hz^−1/2 and a capacitance sensitivity of 0.04 aF Hz^−1/2.

射频反射计可以提供与谐振电路耦合的量子点器件中的电荷和自旋量子位的快速、灵敏的电读出。需要使用电压可调电容器（变容二极管）对谐振器电路进行原位频率调谐和阻抗匹配，以优化读出灵敏度，但传统半导体和铁电基变容二极管的性能在与固体相关的毫开尔文温度范围内显着降低。 -态量子器件。在这里，我们展示了钛酸锶和钽酸钾这两种材料，它们可以在低温下表现出具有大场可调介电常数的量子顺电行为，可用于制造具有完美阻抗匹配和 6 mK 谐振器频率调谐的变容二极管。我们根据电容可调性、耗散损耗和磁场不敏感性来表征 6 mK 的变容二极管。我们使用量子顺电变容二极管来优化碳纳米管量子点器件的射频读出，实现了4.8 μ e Hz ^-1/2的电荷灵敏度和0.04 aF Hz ^-1/2的电容灵敏度。