Electronic structure of superposition states in flux qubits

TL;DR

This paper provides a microscopic analysis of flux qubits, revealing that the superposition states involve surprisingly few microscopic particles, challenging the notion of macroscopic quantum superpositions.

Contribution

It offers a detailed microscopic derivation of flux qubit superpositions, showing they involve fewer particles than previously assumed due to Fermi statistics.

Findings

Superposition states involve a surprisingly small number of electrons.

Large differences in macroscopic observables arise from Fermi statistics.

The analysis challenges the view of flux qubits as macroscopic superpositions.

Abstract

Flux qubits, small superconducting loops interrupted by Josephson junctions, are successful realizations of quantum coherence for macroscopic variables. Superconductivity in these loops is carried by $\sim 10^6$ -- $10^{10}$ electrons, which has been interpreted as suggesting that coherent superpositions of such current states are macroscopic superpositions analogous to Schr\"odinger's cat. We provide a full microscopic analysis of such qubits, from which the macroscopic quantum description can be derived. This reveals that the number of microscopic constituents participating in superposition states for experimentally accessible flux qubits is surprisingly but not trivially small. The combination of this relatively small size with large differences between macroscopic observables in the two branches is seen to result from the Fermi statistics of the electrons and the large disparity between the values of superfluid and Fermi velocity in these systems.