Research On Spectral Purity Of Photon Pair Generated By Spontaneous Parametric Down Conversion

In the development of quantum information,the interference between quantum states plays an important role in quantum teleportation,quantum relaying and so on,which requires that two photons can not be distinguished in any way.Thus,the spectral uncorrelated photons are crucial for the development of quantum information.In this paper,we use spontaneous parameter down conversion(SPDC)to generate counter-propagating photons by adjusting poling period.Since the SPDC process need to satisfy the energy conservation law,the generated photons have frequency correlation.One way to solve this problem is to add filter,but it will decrease the amount of photons.We improve the quality of photons by adjusting the parameter of crystal and pump light.The periodically poled lithium niobate(PPLN)is chosen as down-conversion crystal as it has a higher second-order nonlinear coefficient.And we use quasi-phase matching in this process.Under these basis,the theoretical model of spectral purity under counter-propagating photons is deduced,and the relation between spectral purity and pump period is given.In addition,we discuss and compare the spectral purity with three types of phase matching.The results show that photons generated by the type I phase matching have the highest spectral purity of all the three types.It is also given that the MgO-doped PPLN can improve the spectral purity under the same condition,so it is more suitable to be used in generated photons.It is proved that the purity of generated photons can reach 100% if appropriate parameters are given.Besides,we also compare the purity of counter-propagating and co-propagating scheme,and draw the joint spectral intensity in two cases.Also,the bandwidth of the counter-propagating and co-propagating photons are compared,and the results show the counter-propagating photons have narrower bandwidth.Finally,the model of two-photon is extended to the three-photon case,and the three photons are simultaneously generated by SPDC process.Under the collinear case,and using the birefringence phase matching,the theoretical model of three photons is deduced.The results show that the joint spectral intensity of the three-photon case is an ellipsoid.These simulation results will provide a theoretical basis for generating photons with higher quality.