Levochkina, Anna (2024) Spurious nonlinear processes in traveling wave parametric amplification. [Tesi di dottorato]

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Abstract

A Josephson Traveling Wave Parametric Amplifier (JTWPA) is a nonlinear transmission line-based amplifier that achieves amplification through a parametric process that exploits the nonlinear inductance of Josephson junctions. Due to the highly desirable quantum-limited and broadband characteristics of JTWPAs, such devices are extensively used for microwave signal amplification and are invaluable for a range of applications, including radio astronomy, quantum computing, and quantum sensing. Despite the rapid progress achieved in the development of JTWPAs, there are still considerable obstacles to overcome in order to further optimize their performance and operational modes. A significant challenge associated with JTWPAs is the generation of spurious tones during the operational process. Indeed, JTWPAs’ broadband nature comes with the disadvantage of admitting the activation of spurious nonlinear processes in addition to parametric amplification. These undesired tones, resulting from the intrinsic multi-mode nature of JTWPA devices, have the potential to impair the overall amplifier performance and act as a source of non-ideal added noise. In this thesis, we investigate a prototypical JTWPA device with SNAIL (Superconducting Nonlinear Asymmetric Inductive eLement)-based unit cells, focusing on the amplification behavior along with the spurious tones generation. This specific JTWPA layout is particularly well-suited to investigating the generation of spurious tones re sulting from different nonlinear interactions, as it enables control of the nonlinearity through the application of an external magnetic flux. By combining experimental results and numerical simulations, a comprehensive analysis of spurious tones, such as pump harmonics, parametric instabilities, sidebands, and intermodulation products, is provided. The results of this thesis shed light on the mechanisms behind the generation of JTWPA’s spurious tones, guiding the development of effective strategies to mitigate their impact on amplification performance.

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