Incoherent and Coherent Tunneling of Macroscopic Phase in Flux Qubits

We successfully distinguished flux qubit states by means of single-shot measurement with a high-resolution dc-SQUID. We studied the readout results of the qubit states for two types of sample (classical and quantum) and found that the classical sample exhibited incoherent macroscopic tunneling between two flux states. With the quantum sample, we directly observed the superposition states of the qubit without microwave-spectroscopy experiments.

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