Theory And Experimental Investigation On Realization Of Quantum Logic Gates With Orbital Angular Momentum Of Light In Rb Atoms

With the development of computer technology,the limitations of traditional computer computing speed,energy consumption,integration and other indicators can not meet the computing needs,at this time quantum computers began to appear.The basis of quantum computer is how to realize the operation of single qubit logic gate,two qubit logic gate and multiple qubit logic gate.On the one hand,the nonlinear optical effect of medium is greatly enhanced by the interaction between light and matter,which has great application prospect in the fields of quantum optics,quantum computing,quantum information processing and nonlinear optics.On the other hand,besides the polarization and intensity,the orbital angular momentum(OAM)of the photon opens up new degrees of freedom for the photon.Since the OAM value have an infinite number of discrete values,the infinite OAM value can constitute an infinite Hilbert space.This property enables OAM to carry out high-dimensional quantum state coding in classical optics and quantum optics.Using OAM coding as qubit to realize quantum logic gates has become a new research in quantum information.In this paper,the spatial mode of laguerre-gaussian beam(LG)carrying different orbital angular momentum is encoded to realize two-qubit quantum logic gates by using the good quantum coherence of atomic medium and the transitivity of OAM in the interaction between light and matter.On this basis,the realization process of quantum logic gates in rubidium atomic medium,the spatial properties of quantum logic gates and the regulation of quantum logic gates by light field are emphatically studied.The main research work carried out as follows:First,realization of quantum logic gates in Rubidium atomic three-level system.Firstly,the LG light field mode with different OAM values is coded to realize the control non-gate(c-not)in the process of photonic bandgap four-wave mixing.Secondly,by analyzing the interaction theory between light and atomic medium,the expression of density matrix element of the optical field carrying OAM is deduced,and the experimental scheme is designed by using the transfer law of OAM.In the photonic band gap structure,the output target bit in c-not gate satisfies the phase matching condition in the four-wave mixing process and also satisfies the transfer law of orbital angular momentum.In this paper,the process of four-wave mixing of OAM and photonic bandgap is combined skillfully,and the experimental scheme of quantum logic gate carrying OAM is designed in the structure of photonic bandgap.The experimental platform is built according to the designed experimental scheme,and the experimental results are analyzed.Second,the light modulation of quantum logic gates in three-level systems.Firstly,by changing the intensity of the optical field with different frequencies,it is found that the output signal of the target bit is strongest when the dissonance is zero.With the enhancement of the power of the input field,The signal strength will increase gradually,It is shown that quantum logic gates can be regulated by light field.Secondly,the spatial characteristics of the light signal in the logic gate in rubidium atomic three-level system are analyzed.The spatial characteristics are mainly reflected in the focusing of the logic gate signal and the splitting of the light spots,and the theoretical explanation for the moving,splitting and focusing of the image is given.Third,the realization and phase modulation of the quantum logic gate in rubidium atomic four-level system.By changing the relative angle of the dressing field and the probe field,the output light intensity signal of the quantum logic gate will be enhanced or suppressed by dressing field.This process is called phase control quantum logic gate.Fourth,using the action of microwave field,the transfer characteristics of LG beam in rubidium atomic three-level system are studied.Firstly,the microwave field can effectively regulate the EIT window in the three-level system.Under the effect of the microwave field,the propagation equation of LG beam in the three-level rubidium atomic system is analyzed and deduced.Secondly,by calculating the light intensity information of LG beam,which can be accurately reflected by the OAM value.It is found that the spatial distribution of light intensity varies with the topological charge of LG beam.Finally,the spatial distribution of light intensity can be used to identify OAM number and positive and negative values of topological charge number.