Lahtinen V, Möttönen M
QCD Labs, QTF Centre of Excellence, Department of Applied Physics, Aalto University, FI-00076 Aalto, Finland.
QTF Centre of Excellence, VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland.
J Phys Condens Matter. 2020 Jul 1;32(40). doi: 10.1088/1361-648X/ab98c8.
Superconducting coplanar-waveguide (CPW) resonators are one of the key devices in circuit quantum electrodynamics (cQED). Their performance can be limited by dielectric losses in the substrate and in the material interfaces. Reliable modeling is required to aid in the design of low-loss CPW structures for cQED. We analyze the geometric dependence of the dielectric losses in CPW structures using finite-element modeling of the participation ratios of the lossy regions. In a practical scenario, uncertainties in the the dielectric constants and loss tangents of these regions introduce uncertainties in the theoretically predicted participation ratios. We present a method for combining loss simulations with measurements of two-level-system-limited quality factors and resonance frequencies of CPW resonators. Namely, we solve an inverse problem to find model parameters producing the measured values. High quality factors are obtainable by properly designing the cross-sectional geometries of the CPW structures, but more accurate modeling and design methods for low-loss CPW resonators are called for major future improvements. Our nonlinear optimization methodology for solving the aforementioned inverse problem is a step in this direction.
超导共面波导（CPW）谐振器是电路量子电动力学（cQED）中的关键器件之一。其性能可能会受到衬底和材料界面中的介电损耗的限制。需要可靠的建模来辅助设计用于cQED的低损耗CPW结构。我们使用有损区域参与率的有限元建模来分析CPW结构中介电损耗的几何依赖性。在实际情况中，这些区域的介电常数和损耗角正切的不确定性会在理论预测的参与率中引入不确定性。我们提出了一种将损耗模拟与CPW谐振器的两能级系统限制品质因数和共振频率测量相结合的方法。也就是说，我们解决一个反问题以找到产生测量值的模型参数。通过合理设计CPW结构的横截面几何形状可获得高品质因数，但对于低损耗CPW谐振器，需要更精确的建模和设计方法来实现未来的重大改进。我们用于解决上述反问题的非线性优化方法是朝着这个方向迈出的一步。
