Large Amplitude Harmonic Driving of Highly Coherent Flux Qubits

TL;DR

This paper investigates the behavior of highly coherent flux qubits under large amplitude harmonic driving, revealing complex interference patterns and proposing a new spectroscopic method to better understand their energy spectrum and dynamics.

Contribution

It introduces a Floquet approach beyond two-level models for strongly driven flux qubits and proposes a novel spectroscopic technique using the first excited state.

Findings

Landau-Zener-Stückelberg interference patterns observed

Complex diamond-like patterns emerge at large amplitudes

Higher order diamonds require multi-level descriptions

Abstract

The device for the Josephson flux qubit (DJFQ) can be considered as a solid state artificial atom with multiple energy levels. When a large amplitude harmonic excitation is applied to the system, transitions at the energy levels avoided crossings produce visible changes in the qubit population over many driven periods that are accompanied by a rich pattern of interference phenomena. We present a Floquet treatment of the periodically time-dependent Schr\"odinger equation of the strongly driven qubit beyond the standard two levels approach. For low amplitudes, the average probability of a given sign of the persistent current qubit exhibits, as a function of the static flux detuning and the driving amplitude, Landau-Zener-St\"uckelberg interference patterns that evolve into complex diamond-like patterns for large amplitudes. In the case of highly coherent flux qubits we find that the higher order diamonds can not be simply described relying on a two-level approximations. In addition we propose a new spectroscopic method based on starting the system in the first excited state instead of in the ground state, which can give further information on the energy level spectrum and dynamics in the case of highly coherent flux qubits. We compare our numerical results with recent experiments that perform amplitude spectroscopy to probe the energy spectrum of the artificial atom.