Tunable Superconducting Quantum Bits

The quantum computer has some advantages that the classical computer does not have. The superconducting quantum bit is one of the best solutions to the quantum computer. Superconducting qubits can be compatible with microelectronics technology and has scalability, and its parameters can be adjusted flexibly according to the need. In recent years, a lot of research have been done on the superconducting quantum bits and great achievements have been obtained such as improved coherent time approaching millisecond.In this paper we mainly discussed some characteristics of 3D transmon, which is made up of direct current superconducting quantum interference device (DC SQUID). We built up a measurement system to characterize the transmon, such as the spectrum, Rabi oscillation, the energy relaxation time at ultralow temperature of lOmK. In a three-dimensional cavity, we have successfully realized a tunable transmon with dc SQUID, and its transition frequency changed with the bias voltage and the span of tunable transition frequency is 2.836 GHz.When measuring the flux bias dependent spectrum of the tunable transmon, we have observed phenomenon of avoided energy level crossings. This phenomenon comes from the coupling between transmon and a microscopic two-level system. The experimental results agree well with the simulated results using our theoretical model.