QUANTUM CONTROL AND MEASUREMENT ON FLUXONIUMS
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Superconducting circuit is a promising platform for quantum computing and quantum simulation. A number of efforts have been made to explore the physics in transmon systems and optimize the qubit performance. Compared to transmon, fluxonium is a relatively new type of qubit and attracts more attention recently due to its high coherence time and large anharmonicity. In this thesis, we summarize recent progress toward high fidelity two-qubit gate and readout for fluxonium qubits. We report improved fluxonium coherence either in cavity or cavityless environment. In the former case, we demonstrate single-shot joint readout for two fluxonium qubits and explore various two-qubit gate schemes such as controlled-Z(CZ) gate, controlled-phase(CP) gate, bSWAP gate and cross-resonance(CR) gate. The CZ gate realized by near-resonantly driving the high transitions exhibits 99.2% fidelity from randomized benchmarking. A continuous CP gate set can be implemented by off-resonantly driving the high transitions and shows an average 99.2% fidelity from the cross-entropy benchmarking technique. Other gates involving only computational states are also explored to further improve the gate fidelity, which can take advantage of the high coherence of the fluxonium lower levels. In the cavityless environment, we demonstrate fluorescence shelving readout with 1.7 MHz radiative decay rate for the readout transition while maintaining 52 us coherence time for the qubit transition. Our research explores the basic elements for fluxonium-based quantum processors. The results suggest that fluxonium can be an excellent candidate for not only universal quantum computation but also quantum network and quantum optics studies.