Researchers achieve near-perfect symmetry in graphene quantum dots

Quantum dots in semiconductors such as silicon or gallium arsenide have long been considered for hosting quantum bits in future quantum processors. Scientists at Forschungszentrum Jülich and RWTH Aachen University have demonstrated that bilayer graphene has more to offer here than other materials. The double quantum dots they have created are characterised 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 help the realisation of 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 were able to create six silicon-based spin qubits. With graphene, there is still a long way to go. The material, which was first isolated in 2004, is attractive to many scientists, but the realisation of the first quantum bit has yet to come.

Professor Christoph Stampfer of Forschungszentrum Jülich and RWTH Aachen University said the bilayer graphene is a unique semiconductor, as it shares several properties with single-layer graphene and also has some other special features. “This makes it very interesting for quantum technologies,” Stampfer said.

One of these features is that it has a bandgap that can be turned by an external electric field from zero to about 120 milli-electronvolt. The band gap can be used to confine charge carriers in individual areas, so-called quantum dots. Depending on the applied voltage, these can trap a single electron or its counterpart, a hole — basically a missing electron in the solid-state structure. The possibility of using the same gate structure to trap both electrons and holes is a feature that has no counterpart in conventional semiconductors. According to Stampfer, bilayer graphene is still a fairly new material. “So far, mainly experiments that have already been realised with other semiconductors have been carried out with it. Our current experiment now goes really beyond this for the first time,” Stampfer said.

Stampfer 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. “This symmetry has two remarkable consequences: it is almost perfectly preserved even when electrons and holes are spatially separated in different quantum dots,” Stampfer said.

This mechanism can be used to couple qubits to other qubits over a longer distance; the symmetry also results in a robust blockade mechanism which could be used to read out the spin state of the dot with high fidelity. Professor Fabian Hassler of the JARA Institute for Quantum Information at Forschungszentrum Jülich and RWTH Aachen University said the near-perfect symmetry and strong selection rules are attractive not only for operating qubits, but also for realising single-particle terahertz detectors. “In addition, it lends itself to coupling quantum dots of bilayer graphene 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 realising topological quantum computers,” Hassler said.

Image caption: Artist's impression of bilayer graphene with an electron-hole symmetric double quantum dot, where the electron and the hole are in different layers. Image credit: Sebastian Staacks.