Construction Of Multi-qubit Phase Gate Research On Superconduncting Atom And Microwave Photon

With the rapid development of Quantum information and Quantum optics in recent years,Circuit Quantum Electrodynamics(Circuit QED)based on superconducting atoms and superconducting cavities has become a research hotspot.Superconducting atoms are also known as superconducting qubits.They are based on solid state artificial atoms and have the advantages of easy operation and expansibility.The superconducting cavity provides a good magnetic field environment for superconducting qubits and the microwave photons in the superconducting cavity have a long life.Therefore,the superconducting cavity with high quality factors can not only serve as a quantum data bus but also be used for quantum storage.The circuit QED system with superconducting quantum circuit as the unit has the advantages of small dissipation,easy control,flexible device design and high integration,which provides a method for the direct realization of strong coupling between superconducting quantum bit and microwave photon.Building multi-qubit phase gates in the circuit QED system can realize various tasks of quantum computing.This paper mainly studies the implementation of multi-bit phase gate in QED system based on superconducting atom and superconducting cavities.Firstly,the research background and the development status of circuit electrodynamics and multi-bit phase gate are introduced.Secondly,the circuit QED system based on Josephson junction and Jaynes-cummings model are introduced.Finally,two schemes to realize multi-bit phase gate are proposed : in the circuit QED system,superconducting atoms are used as quantum information carriers and microwave photons are used as quantum information carriers to realize multi-qubit phase gates.The main research contents are as follows :(1)We proposed a one-step scheme to realize multi-objective control of multibit phase gates,in which a superconducting atom(four-level structure)is used to control multiple target qubits simultaneously.The innovation points of the scheme are as follows: 1.As the carrier of quantum information,the superconducting atom encodes part of the quantum information on the dressed state of the superconducting atom.Numerical simulation shows that this encoding scheme partially eliminates th e decoherence effect and improves the robustness of the scheme.2.Bell state,GHZ state and cluster state were prepared by multi-bit controlled phase gate.Numerical simulation shows that the proposed scheme can produce high fidelity Bell,GHZ and 4-qutrit cluster states.3.Markov master equation is used to analyze the dynamic process of the dissipative system,and the results show that this scheme has strong robustness to the attenuation rate of the microwave cavity.In addition,the scheme is universal and can be applied to other types of superconducting atoms and perform the same tasks in a wide range of physical systems.(2)We propose a scheme to realize multi-qubit phase gate.In this scheme,multiple single-mode resonators take their vacuum and single-photon Fock states as the carriers of quantum information,and a multi-level superconducting atom plays a media role as the quantum bus,making the indirect interaction between multiple single-mode resonators.The innovation points of the scheme are as follows: 1.We replaced the coupling strength of the constant with the time-dependent coupling strength of the shaping.The numerical results show that the pulse shaping improves the robustness of the scheme,but improves the fidelity and robustness of the phase gate at the cost of a longer gate time.2.In order to study the feasibility of the scheme,we used the master equation to analyze the influence of the energy relaxation rate of the bus excited state and the cavity dissipation rate of the cavity on t he gate fidelity at the last moment.The numerical results show that the fidelity of the scheme is not affected by the energy relaxation of the excited state of the bus and has high robustness.3.Finally,we consider that since there is no selection rule in superconducting atoms,the microwave field and the classical field can couple any two energy levels.This may affect the feasibility of the scheme.Therefore,we propose an optimal detuning compensation method to inhibit unnecessary transitions.Finally,considering the unwanted transition,the fidelity of the phase gate evolves over time in the numerical simulation.The results show that this method can effectively inhibit the unwanted transition and improve the feasibility of the scheme.