Close to Excellent Particle-hole Symmetry in Graphene Quantum Dots

Researchers at RWTH Aachen College and Forschungszentrum Jülich have uncovered essential traits of double quantum dots in bilayer graphene, an more and more promising materials for potential functions in quantum applied sciences. The crew has demonstrated near-perfect particle-hole symmetry in graphene quantum dots, which might result in extra environment friendly quantum info processing. The examine has been printed in Nature. 

Artist impression of bilayer graphene internet hosting a symmetric electron-hole double quantum dot, the place the electron and gap are positioned on the totally different layers. Picture Credit score: Sebastian Staacks

Double quantum dots have been extensively studied in commonplace semiconductor platforms reminiscent of gallium arsenide, silicon or silicon germanium, as they supply a handy solid-state platform for encoding quantum info. The 2D Supplies and Quantum Units Group at RWTH Aachen College has now proven that double quantum dots in bilayer graphene have extra to supply than in different supplies: they permit the conclusion of methods with near-perfect particle-hole symmetry, the place transport happens through the creation and annihilation of single electron-hole pairs with reverse quantum numbers. This leads to robust choice guidelines that can be utilized for high-fidelity readout schemes of spin and valley qubits.

Antiparticles – Additionally Generally known as Holes

In 1931, the British physicist Paul Dirac printed a paper the place he predicted the existence of an “antielectron”. This antiparticle would have the identical mass as an electron however the reverse cost and spin, and a particle-antiparticle pair would annihilate upon interplay. The existence of the antielectron – which was finally named positron – was proved experimentally one yr later. This was the primary prevalence of an antiparticle.

The idea of antiparticles performs a central function in condensed matter physics, the place antiparticles are sometimes known as holes. For instance, the presence (or absence) of symmetry between particle- and hole-states is essential for characterizing topological phases in condensed matter methods. Nevertheless, particle-hole symmetry isn’t anticipated to be current in semiconductors. A noticeable exception is gapped bilayer graphene within the low-energy restrict.

Quantum Dots for Electrons and Holes

“Bilayer graphene is a really distinctive semiconductor,” explains Christoph Stampfer, professor for Experimental Physics at RWTH Aachen College and corresponding creator of the paper. “It shares a number of properties with monolayer graphene, reminiscent of low spin-orbit coupling and a low-energy spectrum that’s completely electron-hole symmetric. This makes it very attention-grabbing for quantum applied sciences. As well as, it has a band hole that may be tuned from zero to about 120 milli-electronvolts by an exterior electrical subject.” 

The band hole permits to create quantum dots in bilayer graphene utilizing gate geometries similar to these utilized in silicon. Nevertheless, due to the small measurement of the hole, these quantum dots will be ambipolar, which means they will lure each electrons and holes, relying on the voltage utilized on the gates. Exploiting this property and the beautiful degree of electrostatic management achieved of their bilayer-graphene gadgets, Stampfer and colleagues have created electron-hole double quantum dots the place every of the dots hosts at most one electron or one gap. In such a system, electrical transport can solely happen if electron-hole pairs with reverse quantum numbers will be constantly created or annihilated. 

Symmetry Nearly Completely Preserved

This reality has two exceptional penalties. First, by a cautious evaluation of the present via the system, the authors have been capable of experimentally show for the primary time the symmetry between electron and gap states in bilayer graphene. They confirmed that the symmetry is sort of completely preserved even when electron and holes are bodily separated into totally different quantum dots. Second, they unveiled that this symmetry results in a powerful and sturdy blockade mechanism within the transport via the system, which might present a dependable read-out scheme for spin and valley qubits.

“This goes past what will be carried out in standard semiconductors or another two-dimensional electron system,” says Professor Fabian Hassler of the JARA Institute for Quantum Data at RWTH Aachen College, and co-author of the paper. “The close to excellent symmetry that we observe in our work and the robust choice guidelines that consequence from this symmetry are very enticing not just for qubit operation, but in addition for implementing single particle tera-Hertz detectors. As well as, it will likely be attention-grabbing to couple bilayer graphene quantum dots with superconductors – two methods the place electron-hole symmetry performs an essential function. These hybrid gadgets may very well be exploited to create environment friendly sources of entangled particle pairs or engineered topological methods, thus bringing us one step additional in the direction of realizing topological quantum computing gadgets.”

The analysis has been reported in Nature. The information supporting the findings and the code used for the evaluation can be found in a Zenodo repository.  Monetary help for the analysis was supplied, amongst others, by European Union’s Horizon 2020 analysis and innovation program (Graphene Flagship) and by the European Analysis Council (ERC) in addition to by the Deutsche Forschungsgemeinschaft (DFG) inside the Cluster of Excellence Matter of Gentle for Quantum Computing (ML4Q).


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