Flux qubits shed a new light on BCS theory and high-$T_c$ superconductivity

TL;DR

This paper uses flux qubits to estimate the density of Cooper pairs, challenging conventional values and implications for high-temperature superconductivity and the interpretation of Josephson qubits.

Contribution

It introduces a microscopic model linking flux qubit data to Cooper pair density, supporting a strong coupling BCS model and revising the understanding of qubit states.

Findings

Cooper pair density is two orders of magnitude lower than accepted values.

Supports the validity of the strong coupling BCS model.

Revises the interpretation of Josephson qubits as superpositions involving single excited Cooper pairs.

Abstract

A simple microscopic model of a small superconducting loop interrupted by Josephson junction (flux qubit) allows to compute from the experimental data of Wal et.al \cite{Wal} an important parameter - the density of Cooper pairs at zero temperature. This density is determined by the cut-off energy in the BCS model and agrees with the original BCS suggestion but is lower by two orders of magnitude than the value accepted in the modern literature. The immediate consequences of this result are: the validity of the strong coupling BCS model, a plausible picture of electrons recombination into Cooper pairs, and a much weaker condition for the appearance of high-temperature superconductivity. Another consequence is that the popular interpretation of Josephson qubits as macroscopic quantum systems is replaced by a picture of qubit states being superpositions of the ground state and the state containing only a single excited Cooper pair.