High-speed readout method for graphene nanodevices

Graphene is well known for its high electrical conductivity, mechanical strength and flexibility. Stacking two layers of graphene with atomic layer thickness produces bilayer graphene, which also has excellent electrical, mechanical and optical properties and is being utilised in next-generation devices such as quantum computers.

However, it is difficult to gain accurate measurements of the quantum bit states. Most research has primarily used low-frequency electronics to overcome this, but for applications that require faster electronic measurements, the need for faster measurement tools is becoming evident. Now, researchers from Tohoku University have developed a high-speed readout technique by using graphene and making improvements to radio-frequency reflectometry.

Radio-frequency reflectometry works by sending radio frequency signals into a transmission line and then measuring the reflected signals to obtain information about samples. But in devices that use bilayer graphene, the presence of significant stray capacitance in the measurement circuit leads to radio-frequency leakage and less-than-optimal resonator properties. While various techniques have been explored to mitigate this, clear device design guidelines are still needed.

Tomohiro Otsuka, an associate professor at Tohoku University’s Advanced Institute for Materials Research, said the researchers used a microscale graphite back-gate and an undoped silicon substrate to circumvent the common shortfall of radio-frequency reflectometry in bilayer graphene.

“We successfully realised good radio-frequency matching conditions, calculated the readout accuracy numerically and compared these measurements with direct current measurements to confirm its consistency. This allowed us to observe Coulomb diamonds through radio-frequency reflectometry, a phenomenon indicating the formation of quantum dots in the conduction channel, driven by potential fluctuations caused by bubbles,” Otsuka said.

The researchers’ proposed improvements to radio-frequency reflectometry will contribute to the development of next-generation devices, such as quantum computers, and the exploration of physical properties using two-dimensional materials, such as graphene. The research findings were reported in the journal Physical Review Applied.

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