Superconducting circuits based on Josephson junctions can exhibit macroscopic quantum coherence, and thus are suitable to construct qubits. Qubits are the basic unit in a quantum computer, which are controllable using the microwave technology. In recent years great attentions have been paid to implementing quantum computing using superconducting circuits. Although there have been many exciting achievements in the field, to implement large scale quantum computing based on superconducting qubits, coherence performance of individual device element must be further improved.Here we introduce the basic concepts of qubits and gate operations. We will describe how to bring up a phase qubit, and in particular how to characterize a tunable superconducting coplanar waveguide resonator. The resonator is made tunable by inserting a superconducting quantum interference device (SQUID) in the middle of the transmission line. A SQUID can be viewed as a nonlinear inductor, and the effective inductance of SQUID can be tuned by changing the magnetic flux through the SQUID loop. A change of the SQUID inductance will change the total inductance of the transmission line, such that we can tune the resonance frequency of the resonator. We have achieved fast tuning of the resonator, and have studied the influence of a two-level defect (TLS) on dynamics of the coupled qubit and resonator. We demonstrate that a tunable resonator can avoid the influence of a TLS by slightly changing its resonant frequency, and thus can serve as a quantum bus or a memory unit. |