Fast And High-fidelity Readout Of Superconducting Qubits

Superconducting quantum system based on superconducting Josephson junction is currently regarded as the one of the most promising systems to realize quantum computing.As is the main focus of my own research during my Ph.D.over the past few years.High-fidelity state initialization,logic gate manipulation,and quantum state readout are critical for quantum computing.However,due to the fragility of quantum systems,quantum computers will inevitably encounter errors during operation.Quantum error correction provides a feasible path to achieve universal fault-tolerant quantum computing.In quantum error correction,since it is necessary to repeatedly measure the state of qubits to monitor the occurrence of errors to determine whether a correction is required,we not only need to achieve high-fidelity readout of qubits,but also to measure as fast as possible,thereby reducing the probability of other new errors occurring during the measurement process.This document is mainly about a series of work carried out during my Ph.D.aiming to realize fast and high-fidelity readout of superconducting qubits,including the following parts:1.In Chapter 2,I describe the basic principles of dispersive readout of superconducting qubits in detail,analyze the relation between readout fidelity and design parameters of the quantum processers.I propose a full solution for optimal design of readout parameters.At the same time,I find that the realization of Purcell filters and quantum-limited parametric amplifiers are crucial for fast and highfidelity readout,which provides motivation for the following works.2.So in Chapter 3,1 design a new band-stop Purcell filter for fast readout.I verify the effectiveness of the filter and optimize the design parameters through simulation.The results of our experiment are also consistent with the simulation results.Finally,by comparing and analyzing the pros and cons of several different designs in the literature,I also propose several new improved designs and verify the improvement through simulation.3.In Chapter 4,I introduce the basic theory of parametric amplifier and its realization in superconducting quantum system.I demonstrate the realization of a broadband amplifier with a gain of more than 15 dB,a bandwidth of more than 600 MHz,and a noise temperature at the quantum-limit level.With this amplifier,I achieve a fast and high-fidelity multi-qubit readout on a multi-qubit quantum processor,with the highest fidelity of 98.14%in a time in range of 300 ns to 500 ns.4.In Chapter 5,I propose a novel shallow-neural-network-based state discriminator for suppressing readout crosstalk.Experiment shows that this new state discriminator can effectively suppress the readout crosstalk and improve the fidelity of multi-qubit joint readout.This discriminator can be directly transplanted to a field programmable gate array platform,which would be extremely useful for rapid state assignment and real-time feedback control.The innovation points of this thesis mainly include:·A method for optimal design of readout parameters for superconducting qubits is proposed.·A band-stop Purcell filter is designed,which can improve the lifetime limit of qubit and the speed limit of the readout.·Broadband parametric amplifiers with off-chip and on-chip coplanar waveguide impedance transformer are implemented for high-fidelity readout.·A novel state discriminator based on shallow neural network is proposed and implemented which can suppress the readout crosstalk effectively and improve the fidelity of multi-qubit joint readout.