Robust strong coupling architecture in circuit quantum electrodynamics

TL;DR

This paper presents a robust circuit design enabling strong and ultrastrong coupling between superconducting flux qubits and resonators, with experimental validation and theoretical modeling of coupling strength.

Contribution

It introduces a method to systematically vary and optimize qubit-resonator coupling strength, achieving ultrastrong coupling and deriving an analytical expression for it.

Findings

Achieved a coupling strength of 655 MHz, 10% of the resonator frequency.

Demonstrated the transition from strong to ultrastrong coupling regimes.

Validated the theoretical model with experimental spectroscopy data.

Abstract

We report on a robust method to achieve strong coupling between a superconducting flux qubit and a high-quality quarter-wavelength coplanar waveguide resonator. We demonstrate the progression from the strong to ultrastrong coupling regime by varying the length of a shared inductive coupling element, ultimately achieving a qubit-resonator coupling strength of 655 MHz, $10\%$ of the resonator frequency. We derive an analytical expression for the coupling strength in terms of circuit parameters and also discuss the maximum achievable coupling within this framework. We experimentally characterize flux qubits coupled to superconducting resonators using one and two-tone spectroscopy methods, demonstrating excellent agreement with the proposed theoretical model.