Measurements of Current-Phase Relationships in Josephson Junctions

Stanford University, Stanford CA

Investigators

Abstract

Non-technical abstract: Quantum devices have exceptional promise for computation, communication, and sensing. To realize this potential, scientists and engineers must find the right physical system in which to implement quantum bits, called qubits, with sufficiently low error rates. Small superconducting devices are one of the most promising candidates for building physical devices that function as qubits. This research seeks to address fundamental questions about the nature of small superconducting devices. The researchers make sensitive magnetic measurements to detect the current flowing through a superconducting device to determine whether it has a conventional superconducting state or an unconventional, "topological" superconducting state. Topological superconducting state are states that host special modes at their boundaries, called Majoranas modes. Qubits based on these Majorana modes could greatly reduce errors compared to other types of qubits. Technical abstract: The proposed operation of topological qubits is based on a special zero-energy modes known as Majorana modes. The objective of this research is to experimentally demonstrate the existence of Majorana modes using a new generation of fast, highly sensitive Superconducting Quantum Interference Device (SQUID) sensors. The two most important properties are the current-phase relation, a fundamental relation that will have conclusive signatures of Majoranas if they are present and stable, and the parity lifetime, which determines how long the Majorana states are stable. The experiments address longstanding questions in conventional (non-topological) mesoscopic superconductivity, such as the behavior of tunable few-mode junctions with perfect transmission. They also address newer, more urgent questions in topological devices, such as in which systems Majorana fermions exist and for how long their properties are stable. The realization and control of Majoranas would have great foundational significance as an experimental instance of engineered particles with non-Abelian statistics and could also enable certain quantum information technologies. The activities additionally foster the development of fast, ultrasensitive magnetic measurement techniques that are relatively noninvasive. The researchers engage actively in community and outreach activities designed to provide role models and experiential learning to science students at all levels. Student and postdoctoral researchers involved in the research are well-trained in nanotechnology, cryogenics, quantum science, and precision measurements, and are prepared for a lifetime of contributions both as educators and as scientists.