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Quantum bits, or qubits, are the building blocks of quantum computers, and one promising way to build them is by trapping single electrons in nanoscale traps called quantum dots—traditionally made from gallium arsenide or silicon. Recent advances have produced high-quality electron systems in zinc oxide (ZnO), a material attractive for qubits and known for strong electron correlations that can produce new physical behavior.

However, researchers had yet to demonstrate electrostatically defined quantum dots in ZnO and explore their behavior.

“Investigating quantum-dot behavior in ZnO required combining advanced materials growth, device fabrication, and low-temperature transport measurements,” explains Tomohiro Otsuka, the lead investigator of an AIMR research team. “Such an experimental setup had not previously been established.”

In a 2024 article, Otsuka and co-workers addressed this question by integrating advanced ZnO materials, nanofabricated gate structures, and low-temperature transport techniques to produce electrostatically defined quantum dots and characterize their electronic transport properties under varying temperatures and magnetic fields¹.

The team achieved the world’s first demonstration of a gate-defined quantum dot in ZnO. Electrical measurements revealed clear Coulomb oscillations and Coulomb diamonds—signatures confirming that electrons were being confined and controlled one by one within the dot, just as in established quantum-dot materials like gallium arsenide and silicon.

The experiments also revealed something unexpected: signatures of the Kondo effect—a phenomenon where a confined electron’s spin interacts with surrounding conduction electrons—appeared regardless of whether the dot held an even or odd number of electrons, departing from the textbook rule that this effect requires an unpaired spin.

“The realization of gate-defined quantum dots in ZnO, a new material platform, is an important step toward semiconductor quantum technologies based on novel materials,” says Otsuka. “In addition, it was very surprising to observe this unconventional Kondo effect—in particular, that it appeared even when the number of electrons in the dot was even. We expect this physics to provide new insights into strongly correlated electron systems.”

Looking ahead, the team hopes ZnO quantum dots will serve as a new platform for both qubit development and the study of strongly correlated electron physics, with further work needed to pin down the mechanism behind the unconventional Kondo effect.

A personal insight from Dr. Tomohiro Otsuka

Looking back, what does this work mean to you, and where do you hope it leads?

We’ve shown that ZnO can host gate-defined quantum dots, but now a whole new material system is open for exploration, with its own rules that don’t always match what we know from GaAs or silicon. The Kondo effect we observed is a good example: we still don’t fully understand why it behaves the way it does, and that uncertainty is, to me, the most exciting part. It means there’s real physics left to uncover here. I hope this work encourages further exploration of ZnO—and other materials—as a place where both new quantum technologies and new fundamental science can emerge together.

(Author: Patrick Han)

This research highlight has been approved by the authors of the original article and all information and data contained within has been provided by said authors.