Department of PhysicsIn the superconducting quantum circuits, a LC oscillator is a main component and Josephson junction gives nonlinearity. Tuning of resonant frequency can be achieved in general by modulating a Josephson inductance of superconducting quantum interference device (SQUID) with magnetic flux. Here, it is proposed to realize tunable capacitor by using metal/semiconductor junction, which can be applied in the superconducting circuit system.
Prior to realization of tunable capacitor, the electron transport at metal/semiconductor junctions is studied with two different interfacial layers, Al2O3 and graphene. The effects of interface states on electrical properties of the junction are studied by observing the change in Schottky barrier. First, the Schottky barrier height of Au/Ni/Al2O3/4H-SiC junction increases compared to that of Au/Ni/4H-SiC junction. It is because the electrostatic potential increases due to dipole effect on spontaneous polarization of 4H-SiC as a separation between metal and semiconductor increases. On the other hand, in case of Au/Graphene/4H-SiC junction, the Schottky barrier height decreases due to the presence of graphene. When the metal and graphene are in contact, there is charge transfer through Au/Graphene interface and then the graphene becomes doped. In addition, dipole is formed at the interface between Au and graphene. As a result, the effective work function becomes reduced, so does the Schottky barrier height.
Based on the understanding of Schottky junction, tunable capacitor is realized by fabricating Au/Cr/Al2O3/Al/Si junction. With thick Al2O3 film, the electron transfer is blocked for the path between Cr and Al and is allowed only through the Al/Si Schottky junction. Then, the electrons can be captured in the Al floating metal. The amount of charge is dependent on the magnitude of voltage pulses and then discrete capacitance values can be defined. This capacitive memory effect of the tunable capacitor using Schottky junction is expected to be used in the superconducting quantum circuit system in respect that it can change the resonant frequency with discrete capacitance.
Among the existing superconducting quantum circuit models, superconducting qubit is the most representative example of LC oscillators. Many superconducting circuit applications have been used to operate the qubits effectively. One promising application in the superconducting circuit QED is Josephson parametric amplifier (JPA). The JPA has been attracted as a device amplifying a signal in quantum-limited regime. It is observed in this dissertation that the JPA and Josephson parametric converter (JPC) which is another kind of JPA can improve the measurement efficiency in superconducting qubit detection. Also, the squeezed state which is another property of the JPA is studied by preparing it with the JPA and amplifying the squeezed signal with the JPC. The phase-dependence of the squeezed state is measured with homodyne setup and is reconstructed visually by using Wigner function.
Finally, it is explored to reconstruct quantum state as a form of density matrix by using quantum state tomography (QST) in the superconducting multi-qubit system. It is important to extract quantum state in quantum information processing and necessary to expand the analysis on multi-qubit system. In this dissertation, the QSTs on two and three qubits are studied. A joint qubit readout method is used to measure an ensemble of the system. Also, Z-axis phase gate by using hyperbolic secant pulse is discussed in two qubit system. By using sech pulse, the phase accumulated during microwave-activated phase (MAP) gate can be controlled and eventually compensated. It is expected that the state fidelity can be improved by controlling phase on the qubit.clos
