Study On The Preparation Methods And Characteristics Of Entanglement States For Two Qubits Systems

In the fields of quantum communication,quantum computation and quantum cryp-tography,quantum entangled states are extremely important information sources,so it is very important to study the preparation process of quantum entangled states and the characteristics of the preparation system.Based on two techniques of entangled state preparation:Fractional stimulated Raman adiabatic passage technique and spontaneous parametric down-conversion,the design of the control pulse in the two-quantum sys-tem,the characteristics of the system and the frequency correlation,entanglement and interference characteristics of entangled two-photon states in the optical system are stud-ied,in this thesis the following four work has been done:Firstly,based on the principle of Fractional stimulated Raman adiabatic passage method,a semi-counterintuitive pulse sequence(HCI)and a semi-intuitive pulse se-quence(HI)are designed in a two-quantum system,which act on a spin 1/2 four-level closed quantum system and realize the preparation of entangled states.In order to bet-ter analyze the advantages and disadvantages of entanglement preparation based on the principle of Fractional stimulated Raman adiabatic passage,based on the principle of ?pulse dynamics method,a ? pulse sequence which can realize entanglement preparation is designed as a comparison.Secondly,through the analysis of simulation experiments,the pulse amplitude and a series of relative phase values of HCl and hi pulse sequences are determined.Through the analysis of the curves of the system state changing with the control pulse action time and the control pulse area,the characteristics of the HCI pulse sequence,hi pulse sequence and ? pulse sequence based on the ? pulse principle are compared in the aspects of the anti-interference strength and the fidelity of the quantum final state of the quantum system under the action of the ? pulse sequence.The numerical expressions between the final state of the system and the control parameters are derived theoretically.Thirdly,in order to analyze the frequency correlation characteristics and entangle-ment characteristics of two-photon pairs generated by spontaneous parametric down-conversion and to better guide the spontaneous parametric down-conversion process in the experiment,so as to improve the brightness of the entangled light source,based on the two-photon joint spectral function,the spectral symmetry of down-converted two-photon and the correlation characteristics of spontaneous parametric down-converted two-photon are studied when pulse light and continuous light are used as the pump source of spontaneous parametric down-conversion,respectively;When pulsed light is used as the source of spontaneous parametric down-conversion,the influence of the bandwidth of pulsed light and the thickness of crystal on the down-conversion joint spectrum;How the parameters are selected results in a frequency correlated,uncorre-lated,two-photon state with special characteristics,these two-photon states with special characteristics have more requirements in practice.Fourthly,in order to better guide the spontaneous parametric down-conversion pro-cess and improve the visibility of interference,in this thesis,the effect of symmetry of phase matching function on the single photon spectral consistency of signal photons and idle photons is studied by mapping,and the effect of symmetry of phase match-ing function on interference visibility of two-photon states is analyzed.The influence of the length of two interference arms of hom interferometer on coincidence counting rate and interference visibility is analyzed theoretically,and the theoretical derivation is verified by making the relationship curve between the length of interference arms and coincidence counting rate.