Quantum dots in semiconductors such as silicon or gallium arsenide have long been considered hot candidates to host quantum bits in future quantum processors. Scientists at Forschungszentrum Jülich and RWTH Aachen University have now shown that bilayer graphene has much more to offer here than any other material. The double quantum dots they create are characterized by nearly perfect electron-hole symmetry that allows for a strong reading mechanism – one of the necessary criteria for quantum computing. The results are published in well-known journals Natural.
The development of powerful semiconductor spin qubits could aid the realization of large-scale quantum computers in the future. However, a quantum dot-based qubit system is currently in its infancy. In 2022, researchers at QuTech in the Netherlands were able to manufacture 6 spin qubits based on silicon for the first time. With graphene, there is still a long way to go. This material, which was first isolated in 2004, is of great interest to many scientists. But the realization of the first quantum bit is yet to come.
“Bilayer graphene is a unique semiconductor,” explained Prof. Christoph Stampfer of Forschungszentrum Jülich and RWTH Aachen University. “It shares some properties with single-layer graphene and also has some other special features. This makes it very attractive for quantum technology.”
One of these features is the band gap which can be tuned by an external electric field from zero to about 120 milli-electronvolts. Band gaps can be used to confine charge carriers to individual areas, called quantum dots. Depending on the applied voltage, this can trap an electron or its partner, a hole – essentially a lost electron in a solid state structure. The possibility of using the same gate structure to trap electrons and holes is a feature that has no counterpart in conventional semiconductors.
“Bilayer graphene is still a fairly new material. So far, most of the experiments that have been done with other semiconductors have been done with it. Our current experiment really goes beyond this for the first time,” said Christoph Stampfer. He and his colleagues have created what’s called a double quantum dot: two opposite quantum dots, each housing an electron and a hole whose spin properties mirror that of the other almost perfectly.
“This symmetry has two remarkable consequences: it is almost perfectly preserved even when electrons and holes are spatially separated in different quantum points,” Stampfer said. This mechanism can be used to pair qubits to other qubits at greater distances. And what’s more, “symmetry results in a very robust blocking mechanism that can be used to read point spin states with high precision.”
“This goes beyond what can be done in conventional semiconductors or other two-dimensional electron systems,” said Prof. Fabian Hassler of the JARA Institute for Quantum Information in Forschungszentrum Jülich and RWTH Aachen University, co-author of the study. “The near-perfect symmetry and strong selection rules are of great interest not only for operating qubits, but also for realizing single-particle terahertz detectors. Moreover, it is suitable for coupling the quantum dots of bilayer graphene with superconductors, two systems in which electron Symmetry holes play an important role. This hybrid system could be used to create an efficient source of entangled particle pairs or an artificial topological system, bringing us one step closer to realizing a topological quantum computer.”
The research results are published in the journal Nature. The data supporting the results and the code used for the analysis is available in the Zenodo repository. This research was funded partly by the European Union’s Horizon 2020 research and innovation program (Graphene Flagship) and by the European Research Council (ERC), as well as by the German Research Foundation (DFG) as part of Matter of Light. for the Quantum Computing (ML4Q) excellence cluster.