Superconducting flux circuits for coherent quantum annealing
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Quantum annealing is a method with the potential to solve hard optimization problems faster than any classical method. In the near term, quantum annealing is particularly appealing due to its low control requirement, relative to gate-based quantum computation. However, despite the fact that large-scale quantum annealers containing more than 5000 qubits have been made commercially available, identifying a quantum advantage for practical problems has remained an elusive target. Amongst other issues, poor coherence is considered the main prohibitive factor for these annealers to take on the quest for quantum advantage. In this thesis, we make progress in realizing a highly coherent quantum annealer, based on superconducting capacitively-shunted flux qubits (CSFQ). First, we are met with the challenge of crosstalk calibration when implementing individual control of the qubits and couplers in the annealer, which is important for exploring novel annealing protocols. Two different methods, relying on the symmetries of the superconducting circuits, are proposed and successfully implemented to tackle this challenge. Second, we experimentally demonstrate long-range correlation in a chain of couplers, which enables effective coupling of qubits over large distances. The coupler chain could be expanded to a coupler network to support high qubit connectivity, a highly desirable feature when embedding practical-scale optimization problems into the annealer hardware. Finally, we evaluate the noise properties of the CSFQ. Coherence time measurements reveal that the dominant noise in the system is intrinsic flux noise in the two control loops of the qubit. Landau-Zener transition, a toy model for quantum annealing, is investigated in a CSFQ, revealing a crossover from the weak to strong coupling to the environment. This crossover regime was not studied before in either theory or experiment, and we present a phenomenological spin bath model to elucidate this regime.
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Xi Dai (2023). Superconducting flux circuits for coherent quantum annealing. UWSpace. http://hdl.handle.net/10012/19414