Low-frequency characterization of quantum tunneling in flux qubits

TL;DR

This paper explores using low-frequency impedance measurement techniques to characterize quantum tunneling in flux qubits, providing a practical method to determine tunneling amplitudes and degeneracy points.

Contribution

It introduces a novel application of impedance measurement technique at low frequencies to analyze quantum tunneling in flux qubits, including in situ degeneracy point detection.

Findings

Low-frequency IMT reveals information about level anticrossing and tunneling amplitude.

Applying ac bias via the qubit allows in situ degeneracy point identification.

The proposed design is feasible with existing technology and has realistic noise tolerance.

Abstract

We propose to investigate flux qubits by the impedance measurement technique (IMT), currently used to determine the current--phase relation in Josephson junctions. We analyze in detail the case of a high-quality tank circuit coupled to a persistent-current qubit, to which IMT was successfully applied in the classical regime. It is shown that low-frequency IMT can give considerable information about the level anticrossing, in particular the value of the tunneling amplitude. An interesting difference exists between applying the ac bias directly to the tank and indirectly via the qubit. In the latter case, a convenient way to find the degeneracy point in situ is described. Our design only involves existing technology, and its noise tolerance is quantitatively estimated to be realistic.