RAISE-TAQS: Topologically-Engineered Graphene Nanoribbon-based Quantum Systems

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Non-technical description: The goal of this project is to make operational quantum bits (qubits) from graphene nanoribbons (GNRs). Qubits are the building blocks of devices that manipulate quantum information for purposes such as cryptography, simulations, and a variety of other computational tasks. Due to the extraordinary properties of quantum mechanics, quantum information devices (such as quantum computers) are expected to perform calculations significantly beyond what conventional classical computers can ever hope to do. GNRs (which are very narrow strips of graphene) are an exciting new platform for creating qubits because they have excellent electronic, thermal, and mechanical properties, and they can be efficiently fabricated from 'the bottom-up' (i.e., molecule-by-molecule) with atomic precision. The main idea of this project is to incorporate qubits into GNRs by causing individual electrons to be trapped at predetermined locations along the backbone of chemically-engineered GNRs. Because GNRs are molecule-scale structures, qubits arranged in this way could potentially yield quantum devices having reduced size, increased density, and structural flexibility that is unmatched by any other qubit platform. A major focus of this project is to synthesize such GNR-based qubits and to characterize them at the atomic-scale using techniques that include scanning tunneling microscopy and atomic force microscopy. Based on the results of these measurements, suitable GNR qubit candidates are chosen for incorporation into devices designed to measure their quantum properties. The viability of GNR-based qubits as a new quantum information platform is evaluated through this procedure. The broader impacts of this project lie in its strong education and outreach components and the fact that it provides high-level scientific training to graduate students, undergraduates, and high school students, preparing them for careers in science, technology, engineering, and math (STEM) fields. Quantum scientists are trained in how to combine chemistry, material science, and engineering in new interdisciplinary ways. The California community college system has additionally been targeted by this project as a useful conduit for recruiting new STEM majors who better represent the demographics of our nation. This project incorporates a realistic plan to partner with a Bay Area community college in order to develop a new quantum information curriculum that is accessible to community college students, that allow them to transfer their credits to the University of California system, and create new opportunities for them to become involved in cutting-edge research. Technical description: The main goal of this project is to evaluate a new type of graphene nanoribbon (GNR) system for suitability as a quantum information device platform. Qubit functionality in GNRs arises from a recent discovery made by the investigators of this research team that GNRs can host topologically-protected interface states that each contain a single electron spin. These topological interfaces can be engineered at the molecular level and provide a new method for controlling electronic and spin behavior in GNR-based nanostructures. A double-interface GNR geometry has been chosen for this project that places two topological interface states in close proximity, thus allowing the resulting singlet/triplet spin configurations to provide qubit functionality (analogous to the singlet/triplet qubit subspace of Si and GaAs double-quantum-dots). Due to graphene's highly desirable electronic, thermal, and mechanical properties, GNR qubits are expected to have long decoherence times, to function at very high packing density, and to be very device-compatible. Established bottom-up synthesis methods additionally allow atomically-precise GNR structures to be fabricated via highly efficient self-assembly processes. This project is performed by an interdisciplinary research team comprised of three investigators with strengths in chemistry, material science, and electrical engineering. F. Fisher heads the project's chemistry-based efforts to design precursor molecules that self-assemble into topological GNR qubits. M. F. Crommie heads material science-based efforts to characterize the local electronic and spin properties of resulting qubit structures using scanned probe microscopy. J. Bokor heads efforts to incorporate suitable GNR qubit systems into quantum devices for functional evaluation. Fundamental questions addressed by this project include whether topological GNR interface states can be coupled into multi-spin qubit centers, initialized into well-defined quantum states, and efficiently read out. The competition between interface state hybridization effects and on-site Coulomb repulsion is explored through atomically-precise manipulation of the GNR chemical structure, thus presenting new physical regimes for quantum spin engineering that are relevant for quantum information applications. New methods of coupling GNR qubits are planned, including ideas for inducing entanglement between qubits and for addressing individual GNR qubits via electronic nanodevices, assessing the viability of GNRs as a quantum information platform.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.