Switching quantum states in germanene on and off

Researchers from Utrecht University and the University of Twente have found a way to actively control quantum states in the ultrathin material germanene. Using the electric field of a scanning tunneling microscope, they can literally switch the so-called topological states in germanene nanoribbons on and off. These states can be compared to the zeros and ones of a regular computer. The study was published on 12 November in Physical Review Letters.

The computers of the future won’t calculate with zeros and ones, but with qubits: quantum bits that can exist in both states at once. In theory, this makes quantum computers extremely fast and powerful. In practice, however, building qubits is a challenge: they are highly sensitive to noise and easily lose their information.

Researchers are therefore searching for materials stable enough to protect these fragile quantum states. Scientists from Utrecht University and the University of Twente now show that ultrathin strips of germanene might be a promising candidate.

From observing to controlling

A year ago, the same team demonstrated that germanene nanoribbons, strips only two to four atoms wide, host special quantum states at their ends. The new study takes this one step further, showing that these states can be actively manipulated.

“We can switch the topological phase of germanene with an electric field,” says physicist Lumen Eek. He works at Utrecht University’s Institute for Theoretical Physics in the research group of Cristiane de Morais Smith. “This is the first time this has been shown to work reversibly in such a material, both in theory and in experiment.”

Single atomic layer

Germanene resembles graphene, the well-known single layer of carbon atoms, but is made of the heavier element germanium. That difference makes it behave slightly differently: because germanium atoms are larger than carbon atoms, the layer forms a subtle wavy pattern. This corrugated structure makes germanene sensitive to electric fields from a scanning tunneling microscope, allowing researchers to control it. 

Qubits in sight

In the lab, the researchers use the extremely fine needle of a scanning tunneling microscope to probe individual atoms. By changing the local electric field, they can switch the quantum state at the end of a nanoribbon between two configurations: on or off, 1 or 0. “You can think of it like the bits in a classical computer,” says Eek.

This level of control makes germanene a promising building block for so-called topological qubits. Such qubits would be far less sensitive to environmental noise, which is one of the biggest obstacles to developing reliable quantum computers. “The states in germanene are topologically protected,” Eek explains. “That means they’re naturally shielded from interference.”

Collaboration within QuMat

The research is part of the Dutch QuMat programme, a national collaboration between universities working on new quantum materials. “In this project we worked closely with the University of Twente,” says Eek. “There’s a lot of exchange of knowledge and ideas. That’s exactly what QuMat aims for: working together on materials that could form the foundation for future quantum devices.”

About Lumen Eek

Lumen Eek is a PhD candidate at Utrecht University’s Institute for Theoretical Physics. His research focuses on topological materials and the role of symmetry in quantum phenomena.