Insufficiency of avoided crossings for witnessing large-scale quantum coherence in flux qubits

TL;DR

This paper examines whether flux qubits demonstrate macroscopic quantum coherence and argues that avoided crossings alone are insufficient as evidence, proposing more rigorous tests for large-scale quantum coherence.

Contribution

It adapts the large-scale quantum coherence framework to flux qubits and critiques the sufficiency of avoided crossings as evidence for macroscopic quantum behavior.

Findings

Contemporary flux qubit experiments may exhibit quantum coherence over 100 times larger than classical regimes.

Avoided crossings are not sufficient evidence for large-scale quantum coherence.

Proposes alternative rigorous witnesses for detecting macroscopic quantum coherence.

Abstract

Do experiments based on superconducting loops segmented with Josephson junctions (e.g., flux qubits) show macroscopic quantum behavior in the sense of Schr\"odinger's cat example? Various arguments based on microscopic and phenomenological models were recently adduced in this debate. We approach this problem by adapting (to flux qubits) the framework of large-scale quantum coherence, which was already successfully applied to spin ensembles and photonic systems. We show that contemporary experiments might show quantum coherence more than 100 times larger than experiments in the classical regime. However, we argue that the often-used demonstration of an avoided crossing in the energy spectrum is not sufficient to make a conclusion about the presence of large-scale quantum coherence. Alternative, rigorous witnesses are proposed.