Analysis of Possible Quantum Metastable States in Ballistic Graphene-based Josephson Junctions

TL;DR

This paper investigates quantum metastable states in ballistic graphene Josephson junctions, modeling their potential and proposing device designs to observe these states, with initial experimental progress reported.

Contribution

It introduces a model for quantum metastable states in graphene Josephson junctions and proposes device configurations to experimentally observe these states.

Findings

Model of the Washboard potential for graphene junctions

Design proposals for devices with on-chip capacitors and suspended graphene

Initial fabrication progress of proposed devices

Abstract

Graphene is a relatively new material (2004) made of atomic layers of carbon arranged in a honeycomb lattice. Josephson junction devices are made from graphene by depositing two parallel superconducting leads on a graphene flake. These devices have hysteretic current-voltage characteristics with a supercurrent branch and Shapiro steps appear when irradiated with microwaves. These properties motivate us to investigate the presence of quantum metastable states similar to those found in conventional current-biased Josephson junctions. We present work investigating the nature of these metastable states for ballistic graphene Josephson junctions. We model the effective Washboard potential for these devices and estimate parameters, such as energy level spacing and critical currents, to deduce the design needed to observe metastable states. We propose devices consisting of a parallel on-chip capacitor and suspended graphene. The capacitor is needed to lower the energy level spacing down to the experimentally accessible range of 1-20 GHz. The suspended graphene helps reduce the noise that may otherwise come from two-level states in the insulating oxide layer. Moreover, back-gate voltage control of its critical current introduces another knob for quantum control. We will also report on current experimental progress in the area of fabrication of this proposed device.