Bilayer graphene unveils robust double quantum dots for quantum computing breakthrough

Scientists at Forschungszentrum Jülich and RWTH Aachen University have made a groundbreaking discovery in the field of quantum computing. While quantum dots in materials like silicon and gallium arsenide have been considered promising candidates for quantum bits, the researchers have demonstrated that bilayer graphene offers even more potential.

The double quantum dots created by the scientists exhibit nearly perfect electron-hole symmetry, which is a crucial requirement for robust read-out mechanisms in quantum computing. This significant development has been published in the prestigious journal Nature.

The advancement of semiconductor spin qubits holds great promise for the realization of large-scale quantum computers. However, current quantum dot-based qubit systems are still in the early stages of development. In 2022, researchers at QuTech in the Netherlands achieved a major milestone by creating six silicon-based spin qubits. Graphene, on the other hand, still has a long way to go in terms of quantum computing. Nonetheless, this material, first isolated in 2004, has captivated numerous scientists due to its exceptional properties. However, the practical implementation of the first quantum bit using graphene has yet to be achieved.

Prof. Christoph Stampfer of Forschungszentrum Jülich and RWTH Aachen University describes bilayer graphene as a unique semiconductor that shares certain characteristics with single-layer graphene while also possessing additional special features. These qualities make it highly appealing for quantum technologies.

One noteworthy feature of bilayer graphene is its tunable bandgap, which can range from zero to approximately 120 milli-electronvolts when subjected to an external electric field. This bandgap enables the confinement of charge carriers within specific regions called quantum dots. Depending on the applied voltage, these quantum dots can trap either a single electron or its counterpart, a hole (essentially, the absence of an electron within the solid-state structure). The ability to utilize the same gate structure to trap both electrons and holes is a unique attribute absent in conventional semiconductors.

Stampfer further explains that bilayer graphene is still a relatively novel material, and previous experiments mainly replicated those conducted with other semiconductors. However, the team’s latest experiment represents a significant breakthrough, surpassing previous achievements. They successfully created a double quantum dot, comprising two opposing quantum dots, each hosting an electron and a hole whose spin properties almost perfectly mirror each other.

The double quantum dots were produced at the Helmholtz Nano Facility, the central technology platform for the production of nanostructures and circuits in the Helmholtz Association. Credit: Forschungszentrum Jülich / Sascha Kreklau

Wide range of applications

Stampfer emphasizes the remarkable consequences of this symmetry, stating that it remains nearly perfect even when electrons and holes are spatially separated within different quantum dots. This property allows for the coupling of qubits over longer distances, surpassing the capabilities of conventional semiconductors or other two-dimensional electron systems. Additionally, the robust blockade mechanism resulting from the symmetry offers a highly accurate method for reading out the spin state of the quantum dot.

Co-author Prof. Fabian Hassler from the JARA Institute for Quantum Information at Forschungszentrum Jülich and RWTH Aachen University adds that the near-perfect symmetry and strong selection rules make bilayer graphene highly attractive for operating qubits and developing single-particle terahertz detectors. Furthermore, the electron-hole symmetry opens up opportunities for coupling quantum dots of bilayer graphene with superconductors. These hybrid systems hold potential for generating efficient sources of entangled particle pairs and creating artificial topological systems, thereby bringing us closer to realizing topological quantum computers.

The findings of this research have been published in the renowned journal Nature. The supporting data and analysis codes are available in a Zenodo repository.

Source: Forschungszentrum Juelich

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