| Quantum information science,as a multidisciplinary field involving physics,mathematics and computer science,provides the means to store,transmit and process information.Quantum entanglement is one of the key concepts in quantum information science,the preparation of entangled sources has been the focus of many scientists.Recently,it has been shown that the four-wave mixing（FWM）process in atomic ensemble is a promising method to generate entangled sources.First,quantum correlated beams generated from FWM have the characteristic of narrow bandwidth and therefore longer coherence time.Secondly,the quantum correlated beams are generated deterministically and there is no issue of detection efficiency.Finally,the FWM process is a（3）nonlinear process that can generate strong gain without using an optical cavity and ensure the spatial multimode nature of the generated beams.Based on these properties,the quantum correlated beams generated from FWM process can be used to improve detection accuracy and implement quantum information protocols.In this thesis,based on the FWM process in atomic ensemble,three related works are accomplished as follows:1.By combining the FWM processes and beam displacer（BD）interferometers,high-dimensional hyperentangled photon pairs with orbital angular momentum（OAM）degree of freedom（DOF）and path DOF are generated.The BD is used to split a Gaussian pump beam with fixed frequency into two for generating a superposition state of two paths.Afterwards,the biphoton state is produced by two parallel spontaneous FWM processes in the 85Rb vapor cell.The existence of path entanglement,two-dimensional OAM entanglement,and three-dimensional OAM entanglement is successfully demonstrated by using different measurement methods and entanglement criterions.2.The OAM with perfect vortex（PV）mode is introduced into FWM process and combined with the concept of multiplexing in optical communication to deterministically generate 11 pairs of entangled beams with PV mode.The PV and Laguerre-Gaussian（LG）modes are detected by balanced homodyne detection.Then the entanglement property of quantum state is described by the positivity under the partial transposition criterion.For the topological charge l ranging from-5 to 5,the entanglement degrees of OAM multiplexed entanglement with PV modes are all larger than that of OAM multiplexed entanglement with LG modes.More importantly,for OAM multiplexed entanglement with PV modes,the degree of entanglement almost does not change with the topological charge.In other words,OAM multiplexed entanglement enhancement and flattening can be achieved by using PV mode.In addition to research on the entanglement of OAM modes with a single topological charge,the entanglement with coherent superposition OAM modes has also been measured to further demonstrate that entanglement enhancement can be achieved by using PV modes.3.A measurement-free all-optical quantum steering swapping（AOSS）scheme is proposed based on FWM process in the 85Rb vapor cell.This scheme does not involve optic-electro and electro-optic conversion and therefore does not limit the bandwidth of quantum steering swapping.Quantum steering,i.e.,Einstein-Podolsky-Rosen（EPR）steering,is a quantum correlation which stands between Bell nonlocality and entanglement.It differs from Bell nonlocality and entanglement in possessing an asymmetric property.Two FWM processes are used to generate two independent pairs of EPR states.The function of Bell-state measurement without detection is implemented by a low-noise parametric amplifier based on FWM process.After AOSS,there is EPR steering between two initially independent optical modes.When the gains of FWM processes are low,one-way EPR steering exists between two optical modes,i.e.,the asymmetric property of EPR steering is observed.As the gains of FWM processes increase,one-way EPR steering can be turned to two-way EPR steering. |
