Researchers at Tohoku University, in collaboration with the National Institute for Materials Science and the University of Tokyo, successfully demonstrated radio-frequency charge sensing in few-electron zinc oxide quantum dots. Published in Physical Review Applied on July 21, 2026, the breakthrough establishes high-frequency reflectometry as a tool for evaluating semiconductor quantum devices.
Fabricating Few-Electron Zinc Oxide Quantum Dots
Semiconductor quantum dots hold promise as building blocks for scalable quantum computers because they trap individual electrons, using their spins to store quantum information. While researchers have extensively studied silicon and gallium arsenide for this purpose, zinc oxide has emerged as an alternative material with a low nuclear spin environment that may help preserve electron spin states, alongside a direct bandgap suited for optical coupling.
Rapidly and accurately reading the charge state of electrons in zinc oxide quantum dots presented a challenge prior to this work. To solve this, the research team fabricated a device containing two target quantum dots alongside a sensor quantum dot that operates as a sensitive electrometer. By connecting the sensor to a radio-frequency resonant circuit, the team achieved high-frequency reflectometry, which speeds up the detection of electron charge changes. Using this integrated hardware, the researchers successfully confined individual electrons and confirmed the formation of a few-electron double quantum dot.
High-Frequency Reflectometry and Charge Sensing
The measurement technique bridges an experimental gap for zinc oxide quantum devices by enabling high-speed evaluation of quantum states in zinc oxide. Associate Professor Tomohiro Otsuka of the Advanced Institute for Materials Research at Tohoku University emphasized the necessity of rapid readout technologies.
Otsuka further noted that the demonstration establishes an important measurement technique for the material. The current flowing through the target quantum dot and the sensor quantum dot behaves identically when measured alongside radio-frequency reflectometry results, verifying the utility of the sensor circuit.
“By demonstrating high-frequency reflectometry in zinc oxide, we have established an important measurement technique for high-speed evaluation of quantum states in this unique material.”
Tomohiro Otsuka, Associate Professor at the Advanced Institute for Materials Research (WPI-AIMR), Tohoku University
Bridging the Experimental Gap for Quantum Devices
The configuration of target quantum dots alongside a sensor quantum dot creates a foundation for investigating and operating spin qubits in new semiconductor materials. Researchers now possess a high-speed measurement platform to study fundamental properties such as spin relaxation and coherence times.
This platform brings experimental physics closer to realizing high-performance quantum devices built on alternative semiconductors. The study detailing these findings was published online in Physical Review Applied on July 21, 2026, with additional reporting highlighting the advance in late September.
“We now have a high-speed measurement platform that will allow us to investigate fundamental spin properties, including spin relaxation and coherence times, bringing us closer to realizing high-performance quantum devices based on new semiconductor materials.”
Tomohiro Otsuka, Associate Professor at the Advanced Institute for Materials Research (WPI-AIMR), Tohoku University
Next Steps for Tohoku University and Collaborators
The next phase of work involves demonstrating electron spin readout and manipulation within zinc oxide quantum dots.
Researchers also plan to measure key properties including spin relaxation and coherence times using the integrated device structure.