Pairing of Cooper pairs in a Josephson junction network containing an impurity

TL;DR

This paper demonstrates how embedding a quantum impurity in a Josephson network can induce 4e superconductivity, with stable phases described by a boundary double Sine-Gordon model, relevant for quantum coherence applications.

Contribution

It introduces a method to induce Cooper pair pairing in Josephson networks via a quantum impurity, characterized by a boundary double Sine-Gordon model, and explores its implications for quantum systems.

Findings

Induction of 4e superconductivity through impurity embedding.

Stable phases described by a boundary double Sine-Gordon model.

Emergence of a stable attractive fixed point at flux  for enhanced coherence.

Abstract

We show how to induce pairing of Cooper pairs (and, thus, $4e$ superconductivity) as a result of local embedding of a quantum impurity in a Josephson network fabricable with conventional junctions. We find that a boundary double Sine-Gordon model provides an accurate description of the dc Josephson current patterns, as well as of the stable phases accessible to the network. We point out that tunneling of pairs of Cooper pairs is robust against quantum fluctuations, as a consequence of the time reversal invariance, arising when the central region of the network is pierced by a dimensionless magnetic flux $\phi = \pi$. We find that, for $\phi = \pi$, a stable attractive finite coupling fixed point emerges and point out its relevance for engineering a two level quantum system with enhanced coherence.