Entanglement Purification And Concentration Using Heralded High-fidelity Parity-check Detectors Based On Photonic System

Entanglement purification and entanglement concentration are collectively referred to as entanglement distillation.They are effective methods to deal with the impact of noise on entangled quantum systems and the key unit technology in quantum repeaters.Through entanglement distillation,the maximum entangled state with nonlocal high fidelity can be extracted by using local operation and classical communication between communication parties or quantum network nodes,so as to complete safe and efficient quantum communication.The speed of entanglement distillation directly affects the efficiency of quantum communication,and the reliability of entanglement distillation also directly affects the performance of quantum communication.Therefore,entanglement distillation is called the "engine" of quantum communication.Local parity check measurement is the most commonly used method to realize entanglement distillation.In the entanglement purification and concentration scheme based on parity check,the reliability of parity check will inevitably affect the performance of the scheme.So,it is very important for the basic research of quantum communication to study a reliable and high-fidelity parity-check detector and apply it to the scheme of entanglement purification and concentration.To solve the above problems,the main work of this paper is as follows:1.Using the coupling system of semiconductor quantum dot spin and bilateral optical microcavity and common linear optical elements,an error-detected circuit unit is designed to regulate the interaction between photon and quantum dot spin.Then,a heralded high-fidelity parity-check detector(PCD)is designed by using the error-detected circuit unit,which can be used for the non-destructive parity check measurement of polarization degree of freedom of two-photon system.The error-detected circuit unit is based on the general interaction rules of input photon and quantum-dot-microcavity system.It does not require the system to strictly satisfy the specifically ideal conditions,and has high experimental feasibility.PCD can work in a failure-heralded and fidelity-robust fashion,so it can make a correct judgement on the parity mode of a two-photon system and has no destruction on the entanglement degree and fidelity of the original entangled state.2.Based on the heralded high-fidelity PCD,the entanglement purification and concentration schemes for nonlocal polarization entangled photon pairs are proposed,and the efficiency,experimental feasibility and advantages of the schemes are analyzed.The entanglement purification scheme can be used to extract the maximum entangled photon pairs with high fidelity from the mixed entanglement ensemble,and the entanglement concentration scheme can be used to extract the maximum entangled photon pairs from the pure state ensemble without maximum entanglement,so as to achieve the purpose of entanglement distillation.Based on the heralded high-fidelity PCD,the schemes will not reduce the fidelity and entanglement degree of the final entangled state.Under the existing experimental conditions,the schemes have a high probability of success,and the efficiency can be further improved by cyclic iteration.Therefore,compared with the existing schemes,the schemes in this paper have better performance,and have potential application prospects and value in quantum repeater and long-distance quantum communication.