Tunable Coupler in Superconducting Circuits and Impedance Engineering for Josephson Parametric Amplifier

Abstract

In this thesis, we study two topics on superconducting circuits. The first topic is the tunable coupler for resonators. This coupler is implemented with a superconducting circuit coupling with resonators in the dispersive regime. It possesses a high on/off ratio and the coupling between two degenerate resonators can be turned off by tuning the frequency of the coupler via the biasing flux. We present the design, the simulation, and the fabrication of the chip implementing this coupler. We derive the Hamiltonian of the chip via formal circuit quantization. We also present numerical results of the eigenenergies and the eigenstates of the chip. From the numerical results, we show that the tunable coupling and the turn-off point can be realized on the chip.

The second topic is impedance engineering for enhancing the bandwidth of a Josephson parametric amplifier. By transforming the input impedance seen by the Josephson parametric amplifier, the frequency dependence of the gain function can be decreased and the bandwidth of the Josephson parametric amplifier can be enhanced. We study impedance engineering for two different types of flux pumping Josephson parametric amplifier. For each type of amplifier, we develop a corresponding method of impedance engineering, and study implementing the method with transmission lines.