Superconducting lumped-element travelling-wave parametric amplifiers

Doctoral thesis, 2024

Modern information technology requires the development of a set of advanced tools for generation, processing, and detection of weak electromagnetic signals. An important part of such a toolbox is the use of extremely sensitive low-noise amplifiers. In superconducting quantum computers, for example,  he signals typically only contain a few microwave photons, and to amplify such weak signals the noise added by the amplifier should also contain no more than a few photons. Ideally a suitable amplifier should only add the fundamentally required half a noise photon, also known as the quantum limit. Furthermore, the amplifier should not output anything but the amplified signal. Conventional semiconducting amplifiers add up to 10-40 noise photons, and they output noise in the direction of the signal source. Therefore they are not suitable for quantum computers. Parametric amplifiers, on the other hand, are a known class of theoretically quantum-limited devices. These devices use a parametric resonance effect for signal amplification. In this project we investigate a particular implementation of a parametric amplifier, the travelling-wave parametric amplifier (TWPA). The TWPA employs a nonlinear interaction of electromagnetic waves in a waveguide to amplify weak signals. The device we study operates at microwave frequencies and ultra-low  temperatures, and consists of a chain of hundreds of superconducting tunnel junctions embedded in a transmission line. An established theory predicts a very high gain for such devices, but in experiments the resulting gain is significantly smaller. In this work we investigate the physical reasons for such a discrepancy and propose ways to solve the problem. Our study reveals that one of the major reasons for poor amplifier performance is the generation of parasitic modes that take energy away from the useful modes involved in the amplification process. Another problem is that the amplifier lacks isolation and leaks unwanted modes. Therefore we also investigate the required peripheral circuitry to suppress the leakage and provide isolation.