Studies On Microwave Coherence Effect In Superconducting Quantum Circuits

Quantum coherence and interference effects have been one of the very important topics inquantum optics and laser physics, and many novel quantum phenomena based on coherenteffects have been widely studied by the researchers. On the other hand, the superconductingquantum circuits, which are also named as artificial atoms, have the crucial applications inquantum information and quantum computation in recent years. Superconducting qubit is asolid-state device, which not only is easy to be designed, but also has the properties ofcontrollable parameters and experimental fabrication. As a consequence, the varioussuperconducting qubits have been accurately prepared and extensively applied, and manyquantum optical phenomena could be demonstrated in superconducting quantum circuitsystem, which have the very important values of research and application in the field ofquantum optics.In this paper, we discuss quantum coherence effects in the microwave-driven three-level-type superconducting fluxonium qubit. With a good choice of the experimental parameters,the quantum-optics features of the system have been shown and the discussions of the systemcharacters and the inherent physical mechanisms have been done. The main contents are listedas follows:(1) We have studied the switching of dispersion in the three-level-type superconductingfluxonium circuit. When three microwave fields are applied to three transitions of theconducting fluxonium circuit, respectively, the absorption and dispersion spectra of thesuperconducting circuit system are strongly dependent on the relative phase and the intensityof the external field. When the relative phase is changed from π/2to3π/2, the switching frompositive to negative dispersion arises and at the same time the absorption is nearly equal tozero. Switching from positive to negative dispersion has important applications in thesubluminal and superluminal light propagation. In addition, for the fixed relative phase π/2,the dispersion switching with vanishing absorption could be obtained by adjusting the relativeintensity of classical fields.(2) We have investigated the behavior of coherent population trapping in the Δ-typethree-level superconducting fluxonium circuit. In the physical model of three transitionsrespectively driven by three externally electromagnetic field, the coherent population trappingof the system strongly depends on the applied-field relative phase and intensities. When therelative phase is tuned to0or π, the maximal atomic coherence is present and coherentpopulation trapping occurs. While for the choice of π/2, the atomic coherence becomes weak.Meanwhile, for the fixed relative phase π/2, the value of coherence had decreases as theincrease of Rabi frequency of the external field coupled with two lower levels. We have utilized the dressed-state method to explain the physical mechanism of coherent populationtrapping, which is controlled by the relative phase and intensities of the applied fields.In conclusion, based on the interaction between the microwave field and thesuperconducting fluxonium circuit system, the dispersion switching and coherent populationtrapping effects occur, which are dependent on the relative phase and intensities of the appliedfields. Here we use the easily-manipulated superconducting quantum circuit system, andconsider the feasible physical parameters. Thus, the quantum-optics properties of the systemcould be observed experimentally. Our research can not only be helpful for the deepunderstanding of quantum coherence effects, but also have a understanding for the systemfeatures and optical properties of solid state systems, which may offer the help for quantuminformation processing.