Quantum dots in semiconductors such as silicon or gallium arsenide have long been considered hot candidates to host qubits in future quantum processors. Scientists at Forschungszentrum Jülich and RWTH Aachen University have now shown that bilayer graphene has more to offer here than other materials. The twin quantum dots they created have near-perfect electron-hole symmetry that allows for a robust read-out mechanism — a necessary benchmark for quantum computing. The results have been published in the journal nature.
The development of powerful semiconductor spin qubits could help realize large-scale quantum computers in the future. However, current quantum dot-based qubit systems are still in their infancy. In 2022, researchers at QuTech in the Netherlands managed to create 6 qubits spinning on a silicon basis for the first time. With graphene, there’s still a long way to go. The substance, which was first isolated in 2004, is very attractive to many scientists. But the realization of the first qubit is yet to come.
“Pillar graphene is a unique semiconductor,” explains Professor Christoph Stampher from Forschungszentrum Jülich and RWTH Aachen University. “It shares many properties with single-layer graphene and also has some other special features. This makes it very interesting for quantum technologies.”
One such feature is that it has a bandgap that can be tuned by an external electric field from zero to about 120 meV. The band gap can be used to confine charge carriers to individual regions, so-called quantum dots. Depending on the applied voltage, these things can trap a single electron or its counterpart, a hole—essentially a missing electron in the solid-state structure. The possibility of using the same gate structure to trap both electrons and holes is an advantage that the antibody part does not have in conventional semiconductors.
“Bilayer graphene is still a fairly new material. So far, experiments that have already been done have been done with other semiconductors. And our current experiment now really goes beyond this for the first time,” says Christoph Stampher. He and his colleagues have created what’s called a double quantum dot: two opposing quantum dots, each containing an electron and a hole whose spin properties mirror each other almost perfectly.
Wide range of applications
“This symmetry has two remarkable consequences: it is preserved almost perfectly even when electrons and holes are spatially separated at different quantum dots,” Stampher said. This mechanism can be used to pair qubits with other qubits at a longer distance. What’s more, “symmetry results in a very robust blocking mechanism that can be used to read the spin state of a point with great accuracy.”
“This goes beyond what can be done in conventional semiconductors or any other two-dimensional electronic systems,” says Professor Fabian Hassler of the Gara Institute for Quantum Information at Fürschungszentrum-Julich and RWTH Aachen University, co-author of the study. “Near-perfect symmetry and strong selection rules are very attractive not only for powering qubits, but also for realizing single-particle terahertz detectors. In addition, it lends itself to connecting quantum dots from bilayer graphene with superconductors, two systems in which electron-hole symmetry plays an important role.” , as these hybrid systems can be used to create efficient sources of entangled particle pairs or artificial topological systems, bringing us one step closer to realizing topological quantum computers.”
The research results have been published in the journal Nature. The data supporting the results and the codes used for the analysis are available in the Zenodo repository. The research was funded, among others, by the European Union’s Horizon 2020 research and innovation program (Graphene Flagship) and the European Research Council (ERC), as well as by the German Research Foundation (DFG) as part of the Matter of Light for Quantum Computing (ML4Q) group. Distinguished.