Quantum Storage Of Entanglement In Multiple Degrees Of Freedom Based On Atomic Ensembles

After nearly three decades's development,the theory of quantum mechenics has been widely used in all aspects of natural sicence,which promotes a significant advance in this area.At the same time,quantum mechenics itself has been greatly enriched and developed,and also it has derived out some new cross-disciplines through combining with other disciplines.Under this circumstance,quantum information science emerged at this historic moment.Through using quantum information techniques,absolutely secure quantum communication and quantum computers with computing capacity far beyond the classic computers are expected to be achieved.While long distance quantum communication requires the combination of quantum memory and entanglement swapping to form quantum repeaters,quantum storage of information is also essential for complicated computing and processing in quantum computer.Therefore,quantum memory,which directly determines the feasibility of long distance quantum communication and complex quantum computing,has a key role in quantum information science.In quantum information,entanglement is generally loaded on different degrees of freedom of photons.At present,quantum storage of entanglement based on single degree of freedom has been achieved in different systems.However,how to realize quantum storage of multi-degrees-of-freedom(DOFs)entanglement,thus laying the foundation for realizing versatile,high-capacity quantum communication and quantum computation,is still the research problem in the field of quantum information.This thesis is mainly on the goal of achieving multi-DOFs quantum memory,focuses on the experimental gerneration and storage of different-DOFs entanglement and realization of quantum storing hyperentanglement and hybrid entanglement based on multiple DOFs.These works in this thesis are very useful for realizing versatile,high-capacity quantum communication and quantum computation in the future.The main content of this thesis includes:1.An introduction of generating different-DOFs entanglement sources.We introduced the method of different-DOFs entanglement generation based on atomic ensembles and the key process for entanglement source-spontaneous four wave mixing(SFWM).We expounded the optical circuits for SFWM in ladder-type,double-? type and rhombus-type energy-level configuration.Also,we illustrated the detailed experimental generation of non-classical photon pairs at wavelength of 780 nm-776 nm based on hot atomic ensemble and the verification of their non-classical feature.2.The generation and storage of two-color polarization entanglement in atomic ensembles.Based on successfully generating two-color polarization entanglement in one atomic ensemble,we realized the quantum storage of one photon in second atomic ensemble using an enhanced electromagnetically induced transparency(EIT)protocol while coulpling the other photon at telecom-wavelength in optical fiber for transmission.Our experiment results showed that two-color polarization entanglement can be stored in atomic ensemble with high fidelity.3.The quantum storage of Einstein-Podolsky-Rosen(EPR)entanglement based on DLCZ protocol with true position and momentum entities based on cold atomic ensembles.We generated the EPR entanglement between photon and spin wave in true position and momentum base in one atomic ensemble and stored this entanglement based on DLCZ protocl.The entanglement after storage is verified through quantum ghost imaging and ghost interference experiment.Our results proved the effective storage of EPR entanglement in position and momentum base.4.Quatum memory of entanglement in multi-DOFs(polarization and orbital angular momentum(OAM)DOF)based on atomic ensembles.We realized two experimental processes in this experiment.1.Quantum storage of 2.3 hyperentanglement in multi-DOFs.Through spontaneous Raman scattering process(SRS)in first atomic ensemble with the aid of Mach-Zehnder(MZ)interferometer,hyperentanglement between single photon and atomic spin wave is established.We then stored this single photon in second atomic ensemble and successfully established the hyperentanglement between two atomic memories.2.Quantum storage of hybrid entanglement in multi-DOFs.Through SRS process and the aid of a Sagnac interferometer,we prepared the hybrid entanglement between single photon and atomic spin wave.After storing this hybrid entangled photon,we established hybrid entanglement between two memories.By constructing density matrices,checking violation of Clauser-Home-Shimony-Holt(CHSH)Bell inequality,plotting interference curves,checking entanglement Witness,we successfully demonstrated memory-memory hyperentanglement and hybrid entanglement.The main features and innovations of thesis are:1.We systematically and comprehensively introduced the method of generating entanglement in different DOFs and the key process-SFWM based on ladder-type,double-? type and rhombus-type energy-level configuration.2.We for the first time realized quantum storage of two-color polarization entanglement based on atomic ensembles.We demonstrated the fuction of one node in quantum repeater:storing one photon of entangled pair in one atomic ensemble while transmitting the other photon at telecom-wavelength,thus establishing entanglement between atomic spin wave and telecom photon.Becsuse the low loss of telecom photon transmitting in optical fiber,this scheme is very promising in quantum repeater protocol,and can be used in long distance quantum communication based on fiber in the future.3.For the first time,we achieved quantum storage of EPR entanglement in true position and momentum base in cold tomic ensemble.The EPR entanglement beween single photon and atomic spin wave is directly generated through SRS process in the one ensemble,and then is stored.Our results proved a high fidelity quantum storage EPR entanglement,which can be used in spatially multiplexed quantum information process.4.We achieved quantum storage of hyperentanglement and hybrid entanglement in multi-DOFs.For the first time,we established hyperentanglement and hybrid entanglement between separated memories.This is a versatile platform for demonstrating quantum computing protocols and complex quantum networks.