Single-shot Nondemolition Measurement Of Coupled Superconducting Qubits

Quantum computers are composed of experimentally controllable and measur-able quantum units and possess the inherent properties of quantum mechanics such as quantum-state superposition,interference,and entanglement.It can solve certain prob-lems that classic computers cannot.Superconducting quantum computing stands out in quantum computing due to its flexibility,controllability,and scalability.This the-sis describes a systematic research work on coupled superconducting transmon-type qubits:their principle,design and simulation,fabrication,and measurement.The main focus will be on the realization of the qubit quantum-state measurement.A hardware measurement system is designed and built on an Oxford dilution refrigerator for the superconducting multiqubits which includes three parts of the readout,excitation,and bias lines.The software system for the measurement,named QuLab,is also developed Based upon these,the impedance-transformed parametric amplifiers,necessary for the multiqubit single-shot measurement,are successfully characterized.The results show that they have a bandwidth up to 600 MHz with gain above 15 dB and noise tempera-ture approaching the quantum limit.The qubit state differentiation measurements show the signal-to-noise ratio around 3 and the readout fidelity above 97%and 91%for the ground and first-excited states,respectively.The coupled multibit devices are then sys-tematically studied including the qubit characterization demonstrating individual qubit coherence times in the range of 20?40 microseconds,and successful single-and two-qubit gate operations as well as multiqubit crosstalk calibration and minimization.These results provide a good starting point for the further research on superconducting quan-tum computing,especially quantum simulation of the many-body systems using these superconducting circuits and devices.