Near-perfect particle-hole symmetry in graphene quantum dots


May 03, 2023

(Nanowerk News) Researchers at RWTH Aachen University and Forschungszentrum Jülich have discovered an important characteristic of double quantum dots in bilayer graphene, an increasingly promising material for possible applications in quantum technology. The team has demonstrated nearly perfect particle-hole symmetry in graphene’s quantum dots, which can lead to more efficient processing of quantum information.

The study has been published in Natural (“Particle hole symmetry protects spin-valley blockade in graphene quantum dots”). Artist's impression of a graphene bilayer housing a symmetrical electron-hole double quantum dot, where the electrons and holes are located in different layers Artist’s impression of bilayer graphene housing symmetrical electron-hole double quantum dots, where the electrons and holes are located in different layers. (Image: Sebastian Staacks)

Double quantum dots have been studied extensively in standard semiconductor platforms such as gallium arsenide, silicon or germanium silicon, as they provide a convenient solid state platform for encoding quantum information.

The 2D Materials and Quantum Devices group at RWTH Aachen University has now shown that double quantum dots in bilayer graphene have much more to offer than any other material: they enable the realization of systems with nearly perfect particle-hole symmetry, in which transport occurs through creation. and annihilation of single electron-hole pairs with opposite quantum numbers. This results in robust selection rules that can be used for high-fidelity spin and valley qubit reading schemes.

Antiparticles – also known as holes

In 1931, British physicist Paul Dirac published a paper in which he predicted the existence of “antielectrons”. These antiparticles would have the same mass as the electrons but opposite charges and spins, and the particle-antiparticle pairs would annihilate when they interacted. The existence of the antielectron – which was eventually named the positron – was experimentally proven one year later. This is the first occurrence of an antiparticle.

The concept of antiparticles plays a central role in condensed matter physics, where antiparticles are usually referred to as holes. For example, the presence (or absence) of symmetry between the particle state and the hole state is important for characterizing topological phases in condensed matter systems. However, particle hole symmetry is rarely expected to be present in semiconductors. A visible exception is bilayer graphene with gaps in the low-energy boundary.

Quantum dots for electrons and holes

“Bilayer graphene is a very unique semiconductor,” explained Christoph Stampfer, professor of Experimental Physics at RWTH Aachen University and corresponding author of the paper. “It shares several properties with monolayer graphene, such as low spin-orbit coupling and a perfectly electron-hole-symmetric low-energy spectrum. This makes it very attractive for quantum technology. In addition, it has a bandgap that can be adjusted from zero to about 120 milli-electronvolts by an external electric field.”

The band gap makes it possible to create quantum dots in bilayer graphene using gate geometries very similar to those used in silicon. However, due to the small size of the gap, these quantum dots can be ambipolar, meaning they can trap electrons and holes, depending on the voltage applied to the gate. Taking advantage of this property and the extraordinary degree of electrostatic control achieved in their bilayer graphene device, Stampfer and colleagues have created electron-hole dual quantum dots where each point holds a maximum of one electron or one hole. In such a system, electrical transport can only occur if electron-hole pairs with opposite quantum numbers can be continuously created or destroyed.

Symmetry is almost perfectly preserved

This fact has two remarkable consequences. First, by careful analysis of the currents through the system, the authors have been able to prove for the first time experimentally the symmetry between the electron and hole states in bilayer graphene. They showed that the symmetry is nearly perfectly preserved even when electrons and holes are physically separated into distinct quantum dots. Second, they revealed that this symmetry leads to a strong and robust blockade mechanism in transporting through the system, which can provide reliable reading schemes for spin and valley qubits.

“This goes beyond what can be done in conventional semiconductors or other two-dimensional electron systems,” said Professor Fabian Hassler of the JARA Institute for Quantum Information at RWTH Aachen University, and co-author of the paper. “The nearly perfect symmetry we observed in our work and the strong selection rules resulting from this symmetry are of great interest not only for qubit operations, but also for implementing single-particle tera-Hertz detectors. In addition, it would be interesting to couple the quantum dots of bilayer graphene with superconductors – two systems in which electron-hole symmetries play an important role. These hybrid devices can be exploited to create sources of efficient entangled particle pairs or engineered topological systems, taking us one step further towards realizing topological quantum computing devices.”


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