Characterizations Of Control Errors On Multiqubit Superconducting Quantum Circuits

Utilizing quantum properties such as superposition and entanglement,a quantum com-puter works faster than its classical counterpart in certain computational tasks.In the past 20 years,a variety of physical systems have been proposed to build a quantum computer.Among them,superconducting quantum system is considered to be one of the most promis-ing schemes due to its advantages in preparation,manipulation and large-scale integration.However,there are quite a few challenges towards building a practical quantum computer consisting of a large number of qubits.On one hand,we need to construct a scalable mul-tiqubit control system on the prerequisite of maintaining excellent qubit performance and high control accuracy;on the other hand,we need to find efficient methods to character-ize our multiqubit system when more and more qubits are involved,since direct numerical benchmark becomes inefficient as the qubit number increases.This thesis covers the effort of our group in building a comprehensive experimental system of superconducting qubits,which features modular architectures in both hardware and software.In particular,we have designed the enclosing box and peripheral Gaussian filters for the superconducting quantum device,which are used to isolate the device from high-frequency perturbing noise for an improvement of signal quality.Moreover,a website is built to monitor the refrigerator state,share experimental data and manage microwave com-ponents,which provides convenience for the effective operation of the experiment.Overall,the constructed system is scalable,allowing new features and functions to be included in the future.Based on the aforementioned system,We accomplish lots of quantum control and sim-ulation experiments.Meanwhile,we propose a new scheme to characterize control errors of multiqubit quantum circuits,named Clifford sampling,which is not limited by the number of qubits and the depth of the quantum gates.For the quantum circuits where all two-qubit gates are Cliffords,we first replace all unitary single qubit gates by random Clifford sin-gle qubit gates.Then the circuit,error is defined by the difference between the error-free result calculated numerically and the noisy result obtained by experimental measurement.We validate its effectiveness on a superconducting quantum processor by executing various four-qubit circuit sequences with up to ten layers of two-qubit gates.By bencharmarking the fidelity extracted with quantum process tomography,we find that the proposed scheme can be used to optimize the fidelity of quantum gates,which could be useful as we further improve the integration level of our experimental system.