A transmon qubit realized by exploiting the superconductor-insulator transition

TL;DR

This paper introduces a novel transmon qubit using a superconductor-insulator transition in niobium nitride, aiming to overcome limitations of traditional Josephson junctions for scalable quantum computing.

Contribution

It demonstrates a new weak link fabrication method exploiting the superconductor-insulator transition in niobium nitride to create transmon qubits with improved properties.

Findings

Achieved a transmon with anharmonicity of 235 MHz

Linewidth measured at 15 MHz

Utilizes a planar geometry eliminating interfaces

Abstract

Superconducting qubits are among the most promising platforms for realizing practical quantum computers. One requirement to create a quantum processor is nonlinearity, which in superconducting circuits is typically achieved by sandwiching a layer of aluminum oxide between two aluminum electrodes to form a Josephson junction. These junctions, however, face several limitations that hinder their scalability: the small superconducting gap of aluminum necessitates millikelvin operating temperatures, the material interfaces lead to dissipation, and the sandwich geometry adds unwelcome capacitance for high-frequency applications. In this work, we address all three limitations using a novel superconducting weak link based on the superconductor-insulator transition. By locally thinning a single film of niobium nitride, we exploit its thickness-driven superconductor-insulator transition to form a weak link employing only atomic layer deposition and atomic layer etching. We utilize our weak links to produce a transmon qubit, '$planaron$', with a measured anharmonicity of $\alpha/2\pi = 235$ MHz; at present, the linewidth is $\kappa/2\pi = 15 \mathrm{\: MHz}$. The high superconducting gap of niobium nitride can enable operation at elevated temperatures in future devices, and the fully planar geometry of the weak link eliminates superfluous material interfaces and capacitances. The investigation of small patches of material near the SIT can shed new light on the nature of the transition, including the role of dissipation and finite-size effects.