Quantum crossover in moderately damped epitaxial NbN/MgO/NbN junctions with low critical current density

TL;DR

This paper reports on the fabrication and characterization of high-quality epitaxial NbN/MgO/NbN Josephson junctions with low critical current densities, revealing macroscopic quantum phenomena at very low temperatures and potential for quantum experiments.

Contribution

It introduces a method to create NbN/MgO/NbN junctions with tunable properties and demonstrates their quantum behavior at millikelvin temperatures, advancing superconducting quantum device technology.

Findings

Achieved low critical current densities down to 3 A/cm$^2$

Observed macroscopic quantum phenomena below 90 mK

Potential for multiphoton quantum experiments using NbN junctions

Abstract

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d=1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm$^2$ have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN.