Because of their unique electronic properties, symmetric graphene quantum dots have emerged as a promising platform for future qubits and quantum computing. Graphene is a two-dimensional material composed of a single layer of hexagonally arranged carbon atoms. When graphene’s size is reduced to nanoscale dimensions, it forms graphene quantum dots (GQDs), which have discrete energy levels.
Quantum dots in semiconductors like silicon or gallium arsenide have long been thought to be promising candidates for hosting quantum bits in future quantum processors. Scientists from Forschungszentrum Jülich and RWTH Aachen University have now demonstrated that bilayer graphene outperforms other materials in this area. The double quantum dots they have created are characterized by a nearly perfect electron-hole-symmetry that allows a robust read-out mechanism – one of the necessary criteria for quantum computing. The results were published in the journal Nature.
The development of robust semiconductor spin qubits could aid in the future realization of large-scale quantum computers. Current quantum dot-based qubit systems, however, are still in their infancy. For the first time in 2022, researchers at QuTech in the Netherlands were able to create six silicon-based spin qubits. There is still a long way to go with graphene. Many scientists are interested in the material, which was discovered in 2004. However, the first quantum bit has yet to be realized.
Bilayer graphene is a unique semiconductor. It shares several properties with single-layer graphene and also has some other special features. This makes it very interesting for quantum technologies.
Prof. Christoph Stampfer
“Bilayer graphene is a unique semiconductor,” explains Prof. Christoph Stampfer of Forschungszentrum Jülich and RWTH Aachen University. “It shares several properties with single-layer graphene and also has some other special features. This makes it very interesting for quantum technologies.”
One of these characteristics is that it has a bandgap that can be tuned by an external electric field ranging from zero to approximately 120 milli-electronvolts. The band gap can be used to confine charge carriers in specific areas, resulting in quantum dots. Depending on the applied voltage, these can trap a single electron or a hole – essentially a missing electron in the solid-state structure. The ability to trap both electrons and holes using the same gate structure is a feature not found in conventional semiconductors.
“Bilayer graphene is still a fairly new material. So far, mainly experiments that have already been realized with other semiconductors have been carried out with it. Our current experiment now goes really beyond this for the first time,” Christoph Stampfer says. He and his colleagues have 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 remarkable consequences: it’s almost perfectly preserved even when electrons and holes are spatially separated in different quantum dots,” Stampfer explained. This mechanism can be used to connect qubits over a longer distance. In addition, “the symmetry results in a very robust blockade mechanism, which could be used to read out the spin state of the dot with high fidelity.”
“This goes beyond what can be done in conventional semiconductors or any other two-dimensional electron systems,” says co-author Prof. Fabian Hassler of Forschungszentrum Jülich and RWTH Aachen University’s JARA Institute for Quantum Information.
“Not only are the near-perfect symmetry and strong selection rules appealing for operating qubits, but also for realizing single-particle terahertz detectors.” Furthermore, it lends itself to the coupling of quantum dots of bilayer graphene with superconductors, two systems where electron-hole symmetry is important. These hybrid systems could be used to create efficient sources of entangled particle pairs or artificial topological systems, bringing topological quantum computers one step closer to reality.”
