Probing topological effects in multiterminal Josephson junction devices

Northwestern University, Evanston IL

Investigators

Abstract

Non-technical Abstract The sorting of materials into different classes such as solids, liquids and gases helps to clarify our understanding of the properties of these materials and consequently enhances our ability to use them in technologically relevant products. Recently, there has been a major shift in the way materials in condensed matter physics are classified: the properties of many materials can now be understood in terms of their topological classification. Topology is a fundamental characteristic of objects and materials. Two seemingly disparate objects may belong to the same topological class: a donut and a coffee cup may seem to be entirely different objects, but both have a single hole, and hence are topologically similar. Many groundbreaking scientific discoveries of the past century such as the quantum Hall effect have now been reinterpreted in terms of the topological properties of their constituent materials, and in doing so, new insights have been gained. Designing materials and devices with specific topological properties also has potential technological advantages: for example, quantum computers based on topological qubits may be especially resistant to errors due to quantum decoherence. However, synthesizing materials and devices with specific topological properties has proved messy and difficult. This project explores a new way to develop devices with topologically important characteristics using superconducting hybrid devices, devices in which a superconductor is placed in contact with a material like gold or graphene. The properties of such devices mimic those of real crystals, and by appropriate design, analogs of topologically interesting crystals can potentially be fabricated, enabling the study of topologically distinct systems. The devices themselves are fabricated using sophisticated nanolithography techniques and measured at temperatures a few millidegrees above absolute zero. The underlying physics and experimental techniques used in this proposal are applicable to a wide range of scientific research, ensuring that the students engaged in this project will be well trained for future careers in either academia or industry. Technical abstract Materials with topologically non-trivial band structure are being studied intensively at the moment due to their potential applications in a number of areas, including in topological quantum computation. The focus of this project is to create analogs of topologically non-trivial crystals using superconducting hybrid devices, or devices in which superconductors are placed in contact with a normal material such as gold or graphene. The energy levels of quasiparticles in a normal metal in contact with two superconductors depend on the phase difference between the superconductors in much the same way as the energy levels of electrons in a crystal depend on the crystal momentum. With more than two superconductors, analogs of higher dimensional crystals can be fabricated. The goal of this effort is to search for signatures of non-trivial topology in specifically designed devices using electrical transport measurements at millikelvin temperatures, with the devices being fabricated by sophisticated nanolithography techniques. In addition to furthering our understanding of topological systems, the knowledge and understanding gained in this project about the nature of the superconducting proximity effect, particularly in the ballistic devices, will improve the understanding of superconducting correlations in proximity effect devices targeted to topological quantum computing. The nanolithography and low temperature experimental techniques required are similar to the techniques needed in emerging fields in quantum information science, ensuring that students involved in the project are well trained in skills required for the quantum workforce. 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.