Influence Of Controllable Variables On Preparation Of Quantum Entangled States

Quantum entangled states are the transmission carriers of quantum information.In order to make the quality of quantum communication transmission better,scientists are increasingly working on higher quality sources of entanglement.Since the number of photons has limitations,the fundamental method to increase the amount of quantum information is to study the entangled state of preparing more photons.In practical applications,multiphoton interference characteristics are one of the factors affecting quantum operating performance.In essence,interference is indistinguishable of photons.Therefore,the study of the indistinguishability of multiphotons is of great significance.Based on the spontaneous parametric down-conversion technique,this thesis focuses on the preparation of two-photon and multi-photon entangled states,the factors affecting the photon entanglement source and the interference characteristics of photons,the main work is as follows:First,this thesis designs an experimental model of two-photon entangled states.The two-photon frequency entanglement and interference characteristics are obtained by analyzing the combined spectral function of the generated two-photon and the spectrum of the single photon.It studies the influence of pump optical bandwidth and crystal length on two-photon entangled states,and analyzes the effects of two-photon joint spectrum and single-photon spectrum of parametric light.The results show that the two-photon frequency entanglement caused by spontaneous parametric down-conversion decreases with the increase of the pump light bandwidth,and gradually increases with the increase of crystal length.At the same time,the combined spectral intensity decreases.When the crystal length is greater than 10 mm,the line of the two-photon joint spectrum is discontinuous,which will result in a smaller and smaller probability of generating an entangled pair.Therefore,in practical applications,in order to ensure the smooth preparation of the entangled state,the pump light bandwidth should be selected as small as possible,and the crystal length should be appropriately increased within the range of 10 mm.Secondly,it proposes a theoretical model for preparing four-photon entangled states by pumping four pairs of bonded I-cut BBO crystals by pumping light.The results show that this design method improves the multi-photon entangled state and usually requires more complicated optical path evolution process.The photon bunching effect increases the probability that all four photons are output from the same end of the beam splitter by six times more than the four photons are distinguishable in time.This result quantitatively determines the indistinguishability of the four photons.Finally,it proposes a model for preparing an eight-photon entangled state by pumping two Type II cut BBO crystals back and forth.The eight-photon bunching is used to characterize the indistinguishability of the eight-photon.The eight-photon enhancement factor is calculated to be 70.The results show that the photons in the four optical paths are not directly involved in the interaction,but the photons obtained are still entangled photons,which proves the non-locality of the quantum entangled states.Since the photon bunching effect occurs,the probability that all eight photons are output from the same end of the beam splitter is increased by 70 times when the eight-photon is distinguishable in time.This result quantitatively determines the indistinguishability of the eight-photon,which improves the quantum performance compared to the four-photon state.