Research In Superconducting Josephson Qubits About Macroscopic Quantum Phenomena

This thesis reports on our recent experimental research in Nanjing University into superconducting Josesphson qubit, especially the measurement of the superconducting flux qubit. There are four chapters in this paper.First, what is superconducating Josephson qubit? Why doing research in it? Second, how can we measure and control a superconducting quantum qubit? I will introduce the experimental setup and measurement electronics in detail. Third, I will present the main results in measuring the rf-SQUID flux qubit.In Chapter1, all is about the two questions mentioned below. The answers of why doing research in qubit and what is qubit are presented first.In Chapter2, there are two sections.The dilution refrigerator could be introduced first.Then I sum up those must be pay attention in the experiment.such as the noise, the static pulse.etc. In section2.2, there is a detailed list of all the important equipment.In Chapter3, I present the main results in measuring the rf-SQUID flux qubit.In section3.1, I show the sample design of one measured rf-SQUID qubit in experiment. I introduce two measurement methods to identify the qubit signals and the steps of obtaining the basic experimental data,such as potential barrier modulation, energy spectrum, etc.Then in section3.2, macroscopic resonant tunneling phenomena are discussed. We have observed the macroscopic resonant tunneling of magnetic flux between macroscopically distinct quantum states in a superconducting flux qubit. The dependence of the macroscopic resonant tunneling on the barrier height of the potential well, the flux bias, and the initial state were investigated. By using a fast readout pulse, we have measured the probabilities and tunneling rate as functions of flux bias in the rf-SQUID flux qubit by preparing the initial state in one of the double wells. Peaks and dips due to resonant tunneling when the levels in opposite wells line up are observed. We noticed that though around each crossing point, the tunneling rate is enhanced because the energy levels in the double well align with each other, there is not always a maximum. The quantum noise in the qubit leads to the splitting of the middle resonant tunneling peak.In section3.3, the low-frequency flux noise is the topic.We measured the superconducting flux qubit based on Nb/AlOx/Nb Josephson junctions at mK temperature. From the dependence of population in the double potential wells on the external flux, we obtained the low-frequency flux noise S(?)(1Hz) at1Hz. By varying the temperature from49.4mK to400mK, we got that S(?)(1Hz) is a parabolic function of the temperature.In Chapter4, I introduce Landau-Zener transitions at one anti-crossing in multi-level rf SQUID system, and using rate equations to explain the population inversion.