Quantum Coherence in Loopless Superconductive Networks

TL;DR

This study demonstrates long-range quantum coherence in loopless superconductive networks with specific topologies, showing enhanced superconducting properties and phase transition-like behavior, consistent with theoretical models.

Contribution

It provides experimental evidence of quantum coherence in loopless superconductive networks and explores how topology influences superconducting properties.

Findings

Enhanced superconductive gap in specific network branches

Sharp increases in Josephson pair currents

Behavior consistent with phase transition models

Abstract

Measurements indicating that planar networks of superconductive islands connected by Josephson junctions display long range quantum coherence are reported. The networks consist of superconducting islands connected by Josephson junctions and have a tree-like topological structure containing no loops. Enhancements of superconductive gap over specific branches of the networks and sharp increases of pair currents are the main signatures of the coherent states and, in order to unambiguously attribute the observed effects to branches being embedded in the networks, comparisons with geometrically equivalent, but isolated, counterparts are reported. Tuning the Josephson coupling energy by an external magnetic field generates increases of the Josephson currents, along the above mentioned specific branches, which follow a functional dependence typical of phase transitions. Results are presented for double comb and star geometry networks and in both cases the observed effects provide positive quantitative evidence of the predictions of existing theoretical models.