New varactor improves measurements of quantum dot devices at millikelvin temperatures

The development of quantum computing systems relies on the ability to quickly and accurately measure the electrical properties of these systems, such as their underlying charge and spin state. These measurements are typically collected using radio-frequency resonators, tuned using voltage-controlled capacitors called varactors.

Researchers at University College London (UCL) have recently developed a new varactor based on materials exhibiting quantum paraelectric behavior. Their proposed device, presented in a paper published in Natural electronicscan optimize radio frequency readings of quantum dot devices at low temperatures down to a few millikelvins (mK).

“To conduct our research on quantum devices, we use radio-frequency resonators for readout,” Mark Buitelaar, co-author of the paper, told Phys.org. “To optimize this readout, such as tuning the resonator frequencies or coupling them to transmission lines, we needed tunable capacitors, also called varactors, that are robust, insensitive to magnetic fields, and, most importantly, operate at temperatures only a few mK above absolute zero.”

Varactors are widely used in the semiconductor industry, but until now they have not been applied to quantum technologies. This is because they work poorly, if at all, at the very low temperatures at which quantum technologies operate.

In their recent study, Buitelaar and his colleagues set out to develop a new varactor that would work well at these low temperatures. The device they created is based on strontium titanate and potassium tantalate, two materials that exhibit quantum paraelectric properties and field-tunable permittivity at low temperatures.

“Any paraelectric material can be used as a basic component of a varactor, because its permittivity can be adjusted using electric fields, i.e. by simply applying a voltage,” Buitelaar explains. “What makes quantum paraelectric materials such as strontium titanate special is that these paraelectric properties are preserved up to absolute zero.”

Buitelaar and his colleagues evaluated the performance of their varactors in a series of tests and found that they worked extremely well at low temperatures, down to 6 mK. These are the temperatures at which they operate their quantum dot devices.

“Varactors have allowed us to significantly increase our signal-to-noise ratios and thus the accuracy and speed of our measurements,” Buitelaar said. “We believe that our varactors will be of interest to many other researchers who use devices that only operate at extremely low temperatures, such as qubits in semiconductors or superconducting materials.”

In their recent study, the researchers used their varactor to optimize the radiofrequency readout of the carbon nanotube quantum dot devices they developed. Applied to these devices, the varactor achieved a charge sensitivity of 4.8 μe Hz.^−1/2 and a remarkable capacitance sensitivity of 0.04 aF Hz^−1/2.

“In collaboration with colleagues at the London Centre for Nanotechnology at UCL, we are currently working on dopants in silicon as the building blocks of a quantum processor,” Buitelaar added. “Quantum paraelectric varactors certainly help to optimise the measurement accuracy and speed of our quantum state readout, which will be very important as quantum circuits are scaled up to larger systems.”

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