| Superconducting quantum circuits and qubits have the advantages of low dis-sipation,good controllability,design flexibility,and scalability,which have become a promising candidate for the implementation of solid-state quantum computation.They also provide a good platform for the studies of quantum mechanics,atom-ic physics,quantum optics,and quantum simulation.In this thesis,we present systematic studies of superconducting phase qubit,coupled superconducting mul-tilevel and resonator system,and nSQUID-type superconducting qubit.Our main works and results are summarized as follows.(1)Design and fabrication of the superconducting devices.Superconducting qubits and devices are prepared using the micro-and nano-fabrication and multi-layer techniques.High-quality Nb films are first sputter deposited on the cleansed Si substrates as the wiring circuits as well as the coplanar waveguide resonators.Electron beam evaporated Al films and PECVD grown insulatingα-Si layers with e-beam lithographic patterning are then fabricated to form the qubit and SQUID detector loops and capacitors.Finally,high-quality Al/AlO_x/Al Josephson junc-tions are fabricated via the shadow evaporation technique on a Plassys e-beam evaporator.The junctions have the area ranging from 0.2-0.5μm~2and critical current density ranging from 800-2000 mA/μm~2.Coupled superconducting qubit and resonator devices,and nSQUID type qubits are successfully fabricated using these techniques.(2)Coupled superconducting multilevel and resonator systems.Supercon-ducting devices are usually multilevel systems,whose lowest two energy levels are used as qubits.Using the phase device,the properties of the coupled supercon-ducting qudit-resonator system are investigated both experimentally and theoret-ically.Experimentally,we measure the energy spectra of the coupled system via the single-and multi-photon excitation.Theoretically,we propose a model that well describes the experimental results.Furthermore,we use the Lindblad master equation containing various relaxation and dephasing processes to calculate the level populations in the simpler qutrit-resonator system,which allows a clear un-derstanding of the dynamics of the system under the microwave drive.Our results help to better understand various phenomena in which the multilevel effects play a role.(3)nSQUID type superconducting qubits.Two-junction SQUIDs with neg-ative mutual inductance between their two arms,called nSQUIDs,have been proposed for significantly improving quantum information transfer.We have de-signed,fabricated,and characterized the superconducting nSQUID qubits.The two-dimensional(2D)potentials,energy epectrum,Rabi oscillation,energy relax-ation,and Ramsey interference are determined and demonstrated.These results provide clear evidence of the quantum coherence of the device,whose properties are well described by theoretical calculations using parameters determined from spectroscopic measurement.(4)Superconducting phase qubits and nSQUID qubits have the 1D and 2D potentials,respectively,with their potential shapes and energy level spacings tun-able by the applied magnetic flux.Using these two devices,we have investigated the phase transitions between the PT-symmetric phase and PT-broken phase,here PT refers to parity and time-reversal.Also,the macroscopic quantum tunneling phenomenon in the 2D potential landscape is studied. |
