Strong Coupling of a Quantum Oscillator to a Flux Qubit at its Symmetry Point

TL;DR

This paper demonstrates strong coupling between a flux qubit at its symmetry point and an LC oscillator, showing vacuum Rabi oscillations and highlighting the potential for enhanced qubit-resonator interactions while maintaining coherence.

Contribution

The study introduces a flux qubit with tunable gap coupled strongly to an LC oscillator, achieving the highest coupling at the symmetry point with preserved coherence.

Findings

Strong qubit-resonator coupling ($g/h extasciitilde0.1 u_{osc}$) at resonance.

Full spectroscopy of the coupled system.

Observation of vacuum Rabi oscillations.

Abstract

A flux qubit biased at its symmetry point shows a minimum in the energy splitting (the gap), providing protection against flux noise. We have fabricated a qubit whose gap can be tuned fast and have coupled this qubit strongly to an LC oscillator. We show full spectroscopy of the qubit-resonator system and generate vacuum Rabi oscillations. When the gap is made equal to the oscillator frequency $\nu_{osc}$ we find the strongest qubit-resonator coupling ($g/h\sim0.1\nu_{\rm osc}$). Here being at resonance coincides with the optimal coherence of the symmetry point. Significant further increase of the coupling is possible.