EAGER: Braiding of Majorana Zero Modes in the Quantum Hall - Superconductor Hybrids

Duke University, Durham NC

Investigators

Abstract

Non-technical Abstract: Semiconductor electronics currently relies on the flow of electrons and holes, but other types of particle-like "excitations" can be artificially created and controlled. These excitations, which include so-called Majorana fermions and non-abelian anyons, have properties that protect them against external perturbations. As a result of this robustness, control over such "topological excitations" would revolutionize electronics and bring quantum computing closer to reality. Over the past few years, the condensed matter research community achieved dramatic progress in the practical implementation of topological excitations. It was indeed realized that they could be artificially created by inducing superconductivity into certain types of low-dimensional materials such as semiconducting nanowires. The PI proposes an alternative approach to create the topological excitations based graphene subject to high magnetic fields and coupled to superconducting electrodes. The individual topological excitations are connected by gateable links, allowing one to controllably couple and manipulate them. The planar nature of the host material should facilitate making multiple copies of these devices, enabling future scaling and integration with conventional electronics. The project strongly emphasizes mentoring and education: it involves two PhD students, and undergraduate and a high school student. High school students from the NC School of Science and Math will be recruited to participate in the laboratory activities and get a feel for physics research. This research will train the graduate and undergraduate students in the measurement and nanofabrication methods relevant for both industrial and academic careers. Technical Abstract: Working with Josephson junctions based on graphene contacted by type II superconductor, the principal investigator's group reported last year on the first observation of supercurrent through a two-dimensional region in the regime of the quantum Hall effect. This result proves the capability to coherently couple the superconductor electrodes to the quantum Hall edge states. It allows the principal investigator's group to approach superconductor-quantum Hall interfaces with more complex geometries, which are expected to host Majorana fermions. The specific goals are the superconducting electrodes in a shape of quasi-1D "trenches" etched in a quantum Hall mesa, with the edge states counter-propagating on the opposite sides of the contact. It has been predicted that in the spin-polarized quantum Hall regime (for example, at the filling factor equal to one), Majorana zero modes are formed at the ends of the trenches. Realizing these devices requires fundamental understanding of the coupling between the quantum Hall states and superconductor, as well as the development of fabrication techniques to design extremely clean interfaces between the superconducting contacts and the quantum Hall host material. Graphene has much to offer in that regard: a tunable band-structure and a remarkable electronic quality, which results in the appearance of the spin-polarized quantum Hall states at relatively low magnetic fields. Once formed, the Majoranas zero modes could be connected by gateable edge-state links, allowing one to controllably fuse (hybridize) them. Multiple copies of these excitations could be fabricated on the same mesa, enabling their braiding. The planar nature of the host material should facilitate making multiple copies of these devices, enabling future scaling and integration with conventional read-out electronics. Last but not least, the measurements are expected to reveal the fascinating physics of non-abelian anyons, which have properties distinctly different from any conventional quasiparticles. These topological excitations are relevant to a whole class of hybrid topological devices and could lead to breakthroughs that help make quantum computing a reality.