New Tech Improves Quantum Dot Measurements at mK

The advancement of quantum computing depends heavily on precise and rapid measurements of electrical properties, such as charge and spin states. Typically, these measurements use radio-frequency resonators that are adjusted with voltage-controlled capacitors known as varactors.

Researchers at University College London (UCL) have recently introduced a novel varactor designed with materials showing quantum paraelectric behavior. This new device, detailed in a paper published in Nature Electronics, is optimized for radiofrequency readouts of quantum dot devices even at extremely low temperatures, as low as a few millikelvin (mK).

Mark Buitelaar, a co-author of the study, noted, "For our quantum device research, we use radio-frequency resonators to read out data. To refine this readout—such as tuning resonator frequencies or their connection to transmission lines—we required tunable capacitors, or varactors, that are durable, unaffected by magnetic fields, and effective at temperatures only a few mK above absolute zero."

While varactors are common in the semiconductor industry, their use in quantum technologies has been limited due to their poor performance at the extremely low temperatures required by these technologies.

Buitelaar and his team aimed to create a varactor capable of functioning at these low temperatures. Their device, made from strontium titanate and potassium tantalate, demonstrates quantum paraelectric properties with a high field-tunable permittivity at low temperatures.

According to Buitelaar, "Any paraelectric material can serve as a varactor’s core, as their permittivity is adjustable with electric fields. However, quantum paraelectric materials like strontium titanate retain these properties even at temperatures near absolute zero."

Testing showed that their varactors performed exceptionally well down to 6 mK, the operational temperature for their quantum dot devices. "The varactors have greatly improved our signal-to-noise ratio, enhancing the accuracy and speed of our measurements," Buitelaar explained. "We anticipate that these varactors will be valuable to researchers working with devices that function at extremely low temperatures, such as qubits in semiconductors or superconducting materials."

In their study, the team applied the varactor to optimize the radiofrequency readout of carbon nanotube-based 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.

Buitelaar also mentioned ongoing work with colleagues from the London Center for Nanotechnology at UCL on silicon dopants for quantum processors. "The quantum paraelectric varactors are crucial for enhancing the precision and speed of our quantum state readout, especially as we scale up quantum circuits to larger systems," he added.