The Production Of Quantum Entanglement Source And Optimization Of The Spectral Purity Based On PPLN Crystals

Quantum entanglement is a typical feature of quantum communication differing from traditional communication.It attracts more and more attentions of the society and has been developed rapidly in these years.With the continuous improvement of the theory and experiment research,the preparation technology of the entangled source is becoming more perfect,and it has been modified to generate a higher quality of entangled photon pairs.At the same time,the number of the entangled photon is increased as well.These breakthroughs have great significance for the further research and application of quantum communication technology.With the development of the entangled source,the polarization entangled photon pairs are the key point of the research.The major research section of this thesis is the polarization entangled photon.This thesis introduces three parts of the content.First part is the development of the quantum entanglement which contains a brief description of the main theory and experiments of quantum entanglement,and the introduction of some important concepts,mathematical operations of quantum entanglement.The second part and the third part are the emphasis of this thesis.The second part is the preparation of polarization entangled state.In the experiment,the PPMgLN crystal is used to generate the entangled photon pairs via spontaneous parametric down conversion.After that,the photon pairs are measured including the single counting and coincidence counting.The third part is simulation calculation.By using Schmidt decomposition,we calculate the spectral purity of the photon pairs generated via spontaneous parametric down conversion in PPLN crystal with different doping concentration.After the calculation,the relationship between the spectral purity and the parameters of crystal and pump is acquired,so that the parameters can be adjusted to obtain the entangled photon pairs with high spectral purity at telecom-wavelength.At the end of the third part,a new scheme is proposed to reduce the bandwidth of the generated photons which makes the photons have the advantages of high brightness and more suitable for long distance transmission.The main idea of the scheme is to select a appropriate polarization period of the PPLN crystal,in order to make the signal photons and idler photons transmit in opposite directions.By the calculation,we find that the bandwidth of the photons have been significantly reduced,which proves the feasibility.The scheme provides an effective method for the preparation of entangled photon pairs source with narrow bandwidth.