Do experiments based on superconducting loops segmented with Josephsonjunctions (e.g., flux qubits) show macroscopic quantum behavior in the sense ofSchr\"odinger's cat example? Various arguments based on microscopic andphenomenological models were recently adduced in this debate. We approach thisproblem by adapting --to flux qubits-- the framework of large-scale quantumcoherence, which was already successfully applied to spin ensembles andphotonic systems. We show that contemporary experiments might show quantumcoherence more than one hundred times larger than experiments in the classicalregime. However, we argue that the often used demonstration of an avoidedcrossing in the energy spectrum is not sufficient to conclude about thepresence of large-scale quantum coherence. Alternative, rigorous witnesses areproposed.
展开▼
