Apostolidis P, Villis B J, Chittock-Wood J F, Powell J M, Baumgartner A, Vesterinen V, Simbierowicz S, Hassel J, Buitelaar M R
London Centre for Nanotechnology, University College London, London, UK.
Department of Physics and Astronomy, University College London, London, UK.
Nat Electron. 2024;7(9):760-767. doi: 10.1038/s41928-024-01214-z. Epub 2024 Aug 5.
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 μ Hz and a capacitance sensitivity of 0.04 aF Hz.
射频反射测量法能够对耦合到谐振电路的量子点器件中的电荷和自旋量子比特进行快速且灵敏的电学读出。为了优化读出灵敏度，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，但是传统的基于半导体和铁电体的变容二极管在与固态量子器件相关的毫开尔文温度范围内性能会大幅下降。在此我们表明，钛酸锶和钽酸钾这两种材料在低温下能够表现出具有大场可调介电常数的量子顺电行为，可用于制造在6 mK时具有完美阻抗匹配和谐振器频率调谐的变容二极管。我们从电容可调性、耗散损耗和磁场不敏感性方面对6 mK下的变容二极管进行了表征。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 Hz的电荷灵敏度和0.04 aF Hz的电容灵敏度。
