Experiment Study Of Two-photon Entanglement’s Preparation In The Process Of SPDC

In the end of the20thcentury, the human have been in the information society,where information is more and more important in the daily life. Quantum informationis one new subject combined the quantum optics with information science. Quantuminformation’s research are the preparation and operation of quantum states, whichcontains quantum key distribution, entanglement state preparation, quantum phasegate preparation, quantum imaging, quantum teleportation, quantum memory and soon.In the process of quantum information, we can’t do without the quantum statesand its evolution. While single photon state and two photon entanglement state aretwo basic states, which is the basis of the quantum information processing. We have alot of methods to prepare the entanglement state, such as ion trap, CQED,semiconductor material, spontaneous parametric down-conversion (SPDC) and so on.In all these methods, SPDC is more simple and easier to operate. By the non-lineareffect of the crystals, the entanglement state generated by SPDC is more efficient andeasier to operate, which has more vast prospects.(1) Single photon state generated by SPDCIn the SPDC processing, monochromatic pump light and non-linear crystalsinteract with each other, which generates the non-classical field. Every incomingphoton spontaneously produces two down-conversion photons in the SPDC. As thedown-conversion photon owns the time, polarization, frequency, spin entanglement,SPDC processing is often used to generate specific quantum states, which is the basisof the quantum information experiment and its applications.Recently, many schemes have been used to improve the intense of thedown-conversion photons, such as pumping type-I phase-matching crystals, focusingthe pump light, using the resonant cavity to gain down-conversion photons, and so on.Specially, some international experiment groups have used the scheme of pumping theperiodically poled crystals, generating high efficiency down-conversion photons. However, as down-conversion photons have the specific mode and spatial distribution,how to collect the down-conversion photons is the difficult point of research.We have a spontaneous parametric down conversion experiment to generate theparametric photons, and then theoretically analyzed how to efficiently collectparametric photons. In the experiment, we use one405nm laser to pump the type-IBBO non-linear crystal, and collect the parametric photons with the single modefibers. By comparing single counts and the coincidence counts, we will know theefficiency of generating single photon.(2) Biphoton entanglement state generated by SPDCBiphoton entanglement state possesses a specific quantum correlationentanglement character. We supposed that two photons are in the entanglement state.Once known one photon’s state, we will know the other photon’s state cause of theentanglement. Specially, we have operated one photon, and then the other photon willdo the same operation just like the induction with each other. And this entanglementcharacter will not disappear regardless of the distance, which is called the“non-locality”.Recently, many schemes have been used to improve the quality of theentanglement state. Specially, some international groups pump the periodically poledcrystals in the resonant cavity, generating high intensity and narrow line-widthentanglement photons. Moreover, as the requirement of the modern fibercommunication and information safety, how to produce the entanglement photonstransferring in the fiber is the hot point in the research now.As the technology maturity of the fiber communication in the1550nm and810nmwavelength, we ensure that the entanglement states in the fiber communicationwavelength will be more and more prospects. We use the process of SPDC in atwo-BBO type-I crystals geometry, generating a source of polarization-entanglementphoton pairs. In order to verify the generated entanglement states, we measured thedegree of entanglement correlation using one polarization-analyzer system.Furthermore, we have discussed the biphotons function and the interference visibilityin the theoretical analysis.(3) The results of experimentParametric photons are collected by the single mode coupled SPCM and200coincidence counts in100k single counts are collected.We have succeeded in preparing the single photon state and biphoton entanglement state. When the pump laser’s power is12.4mW, the single photon state’ssingle counts are140kcps, while the coincidence counts are19781cps andcoincidence counts rate is14.2%. In the preparation of biphoton entanglement state,pump laser power is300mW, the single counts of810nm is100kcps, while the1550nm counts is485cps, the coincidence counts is200cps or so, and the interferencevisibility is almost90.6%.Then, we will use these prepared single quantum state as the source of thequantum key distribution. And the biphoton entanglement state will be used for thesource of the quantum teleportation.