Superconducting Qubits And Auxiliary Devices: Preparation And Properties

Superconducting quantum computing is based upon superconducting quantum circuits and devices with Josephson junctions as their central elements.Compared with the other quantum computing architecture,superconducting quantum computing has the advantages of extremely low dissipation,good scalability,and flexibility in quantum state preparation,manipulation,and readout.It is therefore a promising candidate in the realization of solid-state quantum computing and quantum information.In this thesis,I present a detailed study of the fabrication processes of superconducting phase qubit,n SQUID qubit,coupled Xmon qubits,and Josephson parametric amplifier(JPA)for the single-shot quantum non-demolition measurement of the qubit quantum states.All the devices are well characterized with optimized parameters and carefully studied.The main results of the thesis can be summarized as follows.(1)Various multilayer preparation processes for the fabrication of superconducting qubits and auxiliary devices are systematically studied.Different processing techniques for the successive growth and patterning of thin films on high-resistivity silicon and sapphire substrates are developed,including dc-magnetron sputtering,e-beam evaporation,thermal evaporation,and plasma enhanced CVD for film growth,and photolithography,direct laser writing,e-beam lithography,reactive ion etching,and wet etching for film patterning.Based upon these techniques,niobium,aluminum,and insulating films(amorphous silicon and calcium fluoride)with required structures,as well as high quality Al/Al Ox/Al Josephson junctions prepared by shadow evaporation are used to successfully fabricate superconducting phase qubit,n SQUID qubit,Xmon qubit,and JPAs.(2)Superconducting phase qubits are successfully prepared.Josephson junctions prepared by shadow evaporation have the area ranging from 0.4 to 0.6?m~2and critical current density from 800 to 2000 m A/cm~2.The phase qubits with small junction sizes thus prepared is found to reduce the qubit decoherence arising from the two-level system.(3)n SQUID qubits are successfully prepared.Different from other kinds of superconducting qubits,n SQUID qubits have double-loop double-junction structure with negative mutual inductance,and have the advantage of fast quantum information transfer.Qubits with Al/Al Ox/Al Josephson junctions having different sizes are fabricated on the same chip using the high-precision multilayer film processing and shadow evaporation techniques.Quantum coherence properties such as Rabi oscillation,energy relaxation,and Ramsey interference are clearly demonstrated for the first time in these devices.(4)Wide-band JPAs are successfully fabricated.The conventional fabrication processes are improved for both the narrow-and wide-band JPAs.The preparation steps are modified and simplified in the new process with shorter preparation cycle and better device reproducibility.The device has a bandwidth up to 600 MHz with gain between 15 and 24 d B and noise temperature approaching the quantum limit,which can be used therefore for the single-shot quantum state measurement of the multiqubit system with the qubit number up to around thirty.(5)Coupled Xmon qubits with the qubit number of 10 are carefully studied.Combined with the fabricated JPAs,the signal-to-noise ratio of qubit non-demolition quantum-state readout is found to be improved significantly.The qubit quantum-state differentiation measurements demonstrate the signal-to-noise ratio around 3 and the readout fidelity above 97%and 91%for the ground and first-excited states,respectively.