Superconducting Qubits, Hybrid Quantum Circuits, And The Associated Detection Technique Using Quantum State Tomography

Quantum computers can solve problems that are insurmountable to classical computers. The core elements of a quantum computer are quantum bits (qubits). There are a lot of physical implementations of qubits. Superconducting circuits based on Josephson junctions become a promising candidate due to the high scalability and low dissipation. Over the past decade, the development of superconducting qubits has been astonishing in terms of the coherence and controllability performance. The coherence time of a single qubit has been raised to the order of100us, which provides the required time for essential quantum gate operations. But to realize the large-scale quantum computing, we still need more ingredients, such as superconducting resonators that can be integrated for hybrid quantum circuits. Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of different systems for better exploring new phenomena and bringing novel quantum technologies.This thesis introduces the basic concepts of quantum computing, including basic operating principles of the three kinds of superconducting qubits:the charge, flux, and phase qubits. It further reviews the development of superconducting qubits, from the perspective of coherence and measurement. In addition, it introduces an experiment as the first step toward utilizing hybrid quantum circuits, which was carried out by us at Zhejiang University. The experiment was characterizing the quantum state of a qubit-resonator hybrid unit using Joint quantum state tomography (QST). The state of a quantum system can be fully represented by a density matrix, including its diagonal and off-diagonal entries. The off-diagonal entries represent quantum phase coherence, the essence of quantum mechanics. To precisely measure the off-diagonal entries, we applied a series of unitary rotations and subsequently a complete set of local measurements. The demonstrated Joint QST technique was proved to be highly accurate, capable of fully characterizing various entangle states. As resonators typically have better coherence performance and more accessible energy levels, the entangled qubit-resonator hybrid can be used as a more effective unit, for advanced quantum information processing.