Calculation Analysis And Application Of Spectral Purity For Parametric Down-conversion Sources

Based on the basic principles of quantum mechanics,quantum information science is becoming the most advanced and active research field in physics and information science with its unique advantages.The heralded single-photon generated by spontaneous parameter down conversion is an important basic resource and widely used in many applications of quantum information processing.One of the most commonly used materials for fabricating entangled photon pairs in current quantum optics experiments is optical superlattices with quasi-phase matching conditions.The generation of single photons with high purity by optical superlattices is one of the hot topics in quantum optics and quantum information science.This paper mainly studies the method to improve the spectral purity of biphoton generated from spontaneous parameter down-conversion by using quasi-phase matching technology.The main content includes the following:(1)Study the method of improving the purity in the forward process.At present,there are two kinds of methods to improve purity: spatial filtering and adjusting the parameters affecting purity.Spatial filtering method will cause brightness reduction and additional noise introduction,so it is imperative to study the other kind of methods.We have systematically analyzed and compared the development of this direction,and improved the corresponding methods.The methods can be summarized into four types: the first method is to obtain factorable photon pairs according to biphoton states;the second method is quantum state engineering method,which uses extended phase matching condition;the third method improves the purity by adjusting the width ratio of the two functions that constitute the joint spectral function;the fourth method is to optimize the pump bandwidth and crystal length.In this paper,we take the periodically-poled potassium titanylphosphate(PPKTP)and periodically-poled lithium niobate(PPLN)crystals as examples to carry out numerical simulation and optimization,and the results show that the four methods can all obtain the purity value of close to 1,and thus can prepare the spectral proximity uncorrelated biphoton source.(2)Study the reverse spontaneous parameter down conversion process.Since the reverse process has the advantages of matching bandwidth and separating entangled photons,the biphoton generated by the reverse spontaneous parameter down-conversion process provides a unique resource for quantum information processing.The relationship between the pump bandwidth and the spectral characteristics in the reverse process is analyzed.For the method to improve the spectral purity of the light source in the reverse process,the group velocity matching method is usually adopted at present.In the forward process,we make corresponding phase matching adjustment by adjusting the four methods that affect the purity factors for the reverse process.We took PPKTP crystal as an example for numerical simulation,and similarly,the purity close to 1 was achieved.In particular,for the type 0 phase matching of the degenerate reverse process,we have obtained a purity of more than 90% in the range of the center wavelength of the parametric light from 1500nm-2000 nm.