Hampus Renberg Nilsson, Chalmers, Quantum Technology

Opponent: Prof. José Aumentado, NIST, USA

In this thesis we explore, theoretically and experimentally, the requirements of developing an ideal low-noise amplifier for amplifying signals close to the quantum limit, such as signals used in superconducting quantum systems. The work is focused on how to enable exponential amplification in a travelling-wave parametric amplifier (TWPA), especially the ones based on three-wave mixing (3WM), although four-wave mixing (4WM) is also treated. These amplifiers are composed of a long chain of cascaded nonlinear inductors and capacitors that form a lumped-element transmission line. The nonlinearity of the inductive element in each unitcell provides the possibility of frequency mixing between the input signal and a strong pump. As a result, some of the pump energy may be transferred to the signal (desired) and to up-converted frequencies (undesired). We extend the theory of the continuous three-mode model for 3WM, both for a discrete chain at frequencies close to the spectral cutoff, as well as for small frequencies with an arbitrary amount of up-converted modes included. In both cases we find that the gain is significantly reduced compared to the prediction by the continuous three-mode model. Based on our findings, we propose a prototype that uses frequencies close to the cutoff frequency to prevent up-conversion, resonant phase matching for phase matching and impedance matching networks for impedance matching. The developed prototype shows very promising results, presented in one of the appended papers, with the possibility of achieving a high gain of ~20 dB in a wide band of several GHz, using only 200 unitcells. Finally we investigate the required peripheral circuitry to suppress leakage and provide isolation.