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, $10\mathrm{%}$ 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.
Highlights
Research over the past few decades has seen significant progress in the field of superconducting quantum circuits [1,2], making such systems the dominant platform for the realization of quantum devices
We demonstrate the linear dependence of qubit-resonator coupling g while increasing the lengthdependent inductance of a shared coupling element
We report on the fabrication and measurement of seven qubit-resonator systems with varying coupling strengths
Summary
Research over the past few decades has seen significant progress in the field of superconducting quantum circuits [1,2], making such systems the dominant platform for the realization of quantum devices. Superconducting qubits as two-level systems, known as “artificial atoms,” are the most researched and robust candidates for various applications in the field of cQED. Employing a multitude of quantum device architectures, the realization of various qubit-cavity coupling regimes has been explored. We experimentally demonstrate a simple, systematic, and robust architecture to achieve strong qubitresonator coupling. We demonstrate the linear dependence of qubit-resonator coupling g while increasing the lengthdependent inductance of a shared coupling element. Using this robust coupling architecture we show how ultrastrong coupling can be achieved without the use of a coupling junction. Our framework is useful for quantum thermodynamic experiments since the heat current is proportional to the square of the coupling between a qubit and the resonator employed for spectral filtering [29,30]
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