Toward better quantum computers: Modeling and design of low-loss devices for circuit quantum electrodynamics

About the speaker

Dr. Valtteri Lahtinen received his doctorate from Tampere University of Technology, Finland, in 2014. In Finland, he has held appointments in Tampere and at Aalto University, where he currently holds a senior-level position of a Research Fellow. Over the years, he has been a visiting researcher in the University of Montreal and Boston University, and currently he is a visiting scientist in the University of Bologna. His research interests and output span from computational electromagnetics and applied mathematics to superconductivity and quantum technology.

Abstract

Due to the associated paradigm shift in computing, a quantum computer holds potential in solving some of the big questions of our time, with potential applications in, for example, medicine research, optimization, and climate modeling. As world-leading tech companies such as IBM and Google, among with a plethora of smaller players, are racing to make this computing paradigm commercially viable, the technological progress has been rapid and is expected to continue.

In this talk, we first introduce the subject of quantum computing on a general level. Then, we focus on one of the leading physical realizations of quantum computing hardware: circuit quantum electrodynamics (cQED) utilizing superconducting microwave circuits. One of the major factors hindering the development of cQED-based quantum computers is decoherence of the superconducting qubits arising from several sources of loss in the circuitry. Particularly, dielectric losses due to two-level-system (TLS) defects occurring in the substrate and in the thin dielectric regions at the material interfaces of the devices potentially limit, e.g., the lifetimes of the quantum bits (qubits) and quality factors of the employed coplanar-waveguide (CPW) resonators. These losses can be simulated utilizing finite-element modeling, but reliable extraction of the relevant material parameters required for accurate modeling is challenging, introducing uncertainty in the simulations.

We discuss the effects of geometric properties and certain fabrication techniques of cQED devices on the TLS-related dielectric losses, utilizing finite-element simulations. Moreover, we study the measurement-based extraction of material parameters of the circuitry by nonlinear optimization, extending our recent work [1]. Finally, we look at some of the novel methods we are currently developing to address these issues.

[1] V Lahtinen and M Möttönen 2020 J. Phys.: Condens. Matter 32 405702