Investigation On Quantum Entanglement Purification For Optical Systems

Quantum information processing employs fundamental principles of quantum mechanics,i.e.,the uncertainty principle and the no-cloning theorem,to ensure unconditional security for quantum information processing.The high-quality entanglement plays a key role in entanglement-based quantum communication.Unfortunately,the fidelity of entanglement will be decreased by the imperfect setups and imperfect manuplations during entanglement generation.In addition,the entanglement will be further polluted by the inherent nosie of quantum channel during entanglement distribution.All these unfavorable issues lead to the maximally entangled state evolve to partial entanglement or mixed entanglement,which further limits the security and reliability of quantum communication.Entanglement purification is a promising method to distill high-quality entanglement from low-quality entanglement assisted by local operation and classical communication.As a result,this paper mainly focuses on the entanglement purification for optical systems.The details are as follows:First,this paper investigates entanglement purification for time-bin entanglement by using sum frequency generation in optical systems to overcome the imperfect deivces and operations during entanglement generation.In addition,the perfect entanglement source is unavailable in current experimental technology.To this end,this paper further proposes a feasiable scheme employing spontaneous parametric down-conversion source to purify time-bin entanglement such as bit-flip errors and phase-flip errors.Ths research findings show that the high-order pairs from spontaneous parametric down-conversion source can be automatically eliminated by the basic principle of sum-frequencey generation requiring two distinct frequency photons arrive at the same time to generate a new photon;Then,this paper investigates the measurement-based entanglement purification in linear optics.For discrete variables,this paper details the analysis for measurement-based entanglement purification by spontaneous parametric down-conversion source.The results indicate that one can effectively wash out the high-order pairs from spontaneous parametric down-conversion source by postselection.For continuous wariables,the analysis of measurement-based entanglement purification is carried out in the cases that the photons are intact and the photon loss happens resulted from dissipation.It is shown that the errors due to the photon loss can be transformed to bit-flip errors and it can be purified in a next step;Next,a measurement-based logical qubit enetanglement purification is proposed to fight against the photon loss because of the fiber attenuation and the environmenet nosie,in which each physical qubit is encoded into a logical qubit by quantum parity code.The research findings show that this logical qubit entanglement purification can still work under the photon loss if the number of photon loss of noisy pairs is less than the tolerance threshold of quantum parity code and the entanglement between different blocks is not destroyed.Moreover,we evaluate the influence of imperfect QND on MBLEPP.It shows that below the error threshold of QND our MBLEPP still enables to purify logical qubit entanglement;Finally,this paper investigates a high-efficient single-copy multiparticle entanglement purification for Greenberger-Horne-Zeilinger(GHZ)states using hyperentanglement,which is more efficient compared with two-copy multiparticle entanglement purification.We purify the polarization entanglement at the cost of time-bin entanglement and the research findings illustrate that the fidelity of resultant state is higher than the maximum of two fidelities before purification.Furthermore,this purification is enxteded to correct the errors of GHZ states with arbitrary number of photons.