Researchers from Germany's RWTH Aachen University, Forschungszentrum Jülich and Japan's National Institute for Materials Science (NIMS) have found that bilayer graphene allows the realization of electron–hole double quantum dots that exhibit near-perfect particle–hole symmetry. Moreover, They showed that particle–hole symmetric spin and valley textures lead to a protected single-particle spin-valley blockade that will allow robust spin-to-charge and valley-to-charge conversion, which are essential for the operation of spin and valley qubits.
Quantum dots in semiconductors such as silicon or gallium arsenide are considered great candidates for hosting quantum bits in future quantum processors. The recent study essentially shows that bilayer graphene has even more to offer than other materials. The double quantum dots the researchers 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.
"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 features is that it has a bandgap that can be tuned 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 counter part 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," 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.
"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. And what’s more, "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 Prof. Fabian Hassler of the JARA Institute for Quantum Information at Forschungszentrum Jülich and RWTH Aachen University, co-author of the study. "The near-perfect symmetry and strong selection rules are very attractive not only for operating qubits, but also for realizing 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 realizing topological quantum computers."