Symmetric graphene quantum dots for future qubits

Quantum dots in semiconductors such as silicon or gallium arsenide have long been seen as hot candidates for hosting quantum bits in future quantum processors. Scientists from Forschungszentrum Jülich and RWTH Aachen University have now shown that bilayer graphene has even more to offer than other materials. The quantum double dots they created are characterized by near-perfect electron-hole symmetry that allows for a robust readout mechanism, one of the necessary criteria for quantum computing. The results were published in the renowned journal Nature.

The development of robust semiconductor spin qubits could aid the realization of large-scale quantum computers in the future. However, current quantum dot-based qubit systems are still in their infancy. In 2022, researchers from QuTech in the Netherlands were able to create 6 silicon-based spin qubits for the first time. With graphene, there is still a long way to go. The material, first isolated in 2004, is highly attractive to many scientists. But the realization of the first quantum bit is yet to come.

“Bilayer graphene is a unique semiconductor,” explains Prof. Christoph Stampfer of the Forschungszentrum Jülich and RWTH Aachen University. “It shares several properties with single-layer graphene and also has some other special characteristics. This makes it very attractive 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 millielectron volts. The band gap can be used to confine charge carriers to single areas, so-called quantum dots. Depending on the voltage applied, these can trap a single electron or its counterpart, a hole, basically a missing electron in the solid-state structure. The ability to use the same gate structure to trap both electrons and holes is a feature not found in conventional semiconductors.

“Bilayer graphene is still a fairly new material. So far, mainly experiments have been carried out with it that have already been carried out with other semiconductors. Our current experiment now really goes beyond this for the first time,” says Christoph Stampfer. He and his colleagues created a so-called double quantum dot: two opposing quantum dots, each housing an electron and a hole whose spin properties mirror each other almost perfectly.

Wide range of applications

“This symmetry has two notable consequences: It is almost perfectly conserved even when electrons and holes are spatially separated at different quantum dots,” Stampfer said. This mechanism can be used to couple qubits to other qubits over a greater distance. And what’s more, “the symmetry results in a very robust locking mechanism that could be used to read the spin state of the dot with high fidelity.”

“This is beyond what can be done in conventional semiconductors or any other two-dimensional electronic system,” says Prof. Fabian Hassler of the JARA Institute for Quantum Information at Forschungszentrum Jülich and RWTH Aachen University, who co-authored the study. “The near-perfect symmetry and strong selection rules are very attractive not only for the operation of qubits, but also for the fabrication of single-particle terahertz detectors. Furthermore, it lends itself to coupling bilayer graphene quantum dots with superconductors, two systems in which electron hole symmetry plays an important role. These hybrid systems could 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 were published in the journal Nature. The data supporting the results and the codes used for the analysis are available in a 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) under the Matter project of Light for Quantum Computing Cluster of Excellence (ML4Q).

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