Realizing Orbital Angular Momentum Continuous-variable Entanglement Based On Atomic Ensembles

Quantum communication is a new research field combining quantum theory and information theory and concerning the national information and national defense security.It is the country's priority for the development of information technology and industrial highlands and has huge applications in finance,government affairs,national defense,and electronic information.The generation of quantum entanglement and the establishment of entanglement channels are the core technologies of quantum communication.In the entangled channel,the alkali nodes act as basic frameworks for the integration,storage,distribution of quantum information as immediate areas of research focus.To realize the efficient direct interaction between the alkali atoms and the light fields it is necessary to generate the entangled states whose wavelength match with the transition frequency of atomic nodes.The non-linear four-wave mixing effect of alkali atoms is a good scheme to generate a twin beam and an entangled state.Compared with other alkali atoms,the hyperfine energy level splittings of ground states of cesium atoms is larger,which reduces the interference of other energy levels.Furthermore,the 895 nm at the ¹³³Cs D1 line lies well within the wavelength regime of the exciton emission from InAs quantum dots,which is the bond between the atomic and solid-state systems to realize the exchange of quantum states.The interaction between the cesium atomic ensembles as effective nodes and the corresponding non-classical states provides a potential resource for coherent interfaces in the quantum information processes.The two-mode squeezed states generated on the nondegenerate four-wave mixing in alkali atoms can be directly devoted to atom-based quantum communication and quantum computing because their optical frequency corresponds to the transition energy level of atoms.At the same time,the non-classical states generated by the four-wave mixing process without the use of a cavity has prompted our interest in observing arbitrary spatial modes,which increases the amount of information contained in the field,making the multi-spatial-mode non-classical entangled states have developed rapidly in quantum images entanglement,quantum imaging and broad-bandwidth high-dimensional quantum memory.The orbital angular momentum?OAM?beam as a special multi-spatial-mode beam with special intensity distribution and phase modes can be applied to the optical manipulation of microscopic particles domain.The high dimension of OAM beams can also be used for high-capacity communication and high-speed information processing.In this thesis,we generate highly correlated twin beams using the four-wave mixing process and research the OAM continuous-variable entanglement.The main contents are as follows:In Chapter 1,we first introduce several basic quantum states:coherent state,squeezed state,and entangled state.The description of these quantum states is confined to a single mode.However,the quantum state with multiple-spatial-mode is currently a popular research field.In particular,non-classical states with orbital angular momentum have attracted much attention.Therefore,we have also introduced the basic knowledge of orbital angular momentum light beams.In the second chapter,we describe the basic theoretical model of double-?43?scheme four-wave mixing and exhibit the theoretical expression of the two-mode squeezed state in four-wave mixing process.The effects of the gain and the internal loss of the system on the two-mode intensity-difference squeezing are also considered.In Chapter 3,we demonstrate quantum correlated twin beams at the ¹³³Cs D1 line using the two locked beams as the pump and probe by an optical phase-locked loop?OPLL?to get the intensity difference squeezing of 6.0 dB at the analysis frequency of 0.23 MHz.Finally,we analyze the dependence of the four-wave mixing gain and intensity difference squeezing on various experimental parameters to obtain the best experimental parameters for the highest degree.In Chapter 4,using the electro-optical modulator?EOM?to generate the probe,optimizing the mechanical stability of the compact quantum light source and the electronic noise of the detection system,and selecting appropriate experimental parameters,an intensity-difference squeezed state with a frequency as low as 0.7 kHz and a maximum squeezing of 6.5 dB.is obtained for the first time.In the fifth chapter,orbital angular momentum continuous-variable entanglement with the degree of 4 dB is obtained based on four-wave mixing in cesium atomic ensembles.The experimental research on orbital angular momentum continuous-variable entanglement has laid a foundation for the study of multipartite entanglement.The innovations are as follows:?.The intensity-difference squeezing with the wavelength of 895 nm is improved to 6.5 dB,which can be mainly attributed to very good frequency and phase-difference stability between the pump and probe in the four-wave maxing process by using OPLL and EOM,respectively.The pump-probe frequency difference is 9.2 GHz corresponding to the hyperfine splitting of ground state 6S_1/2 in cesium.?.We demonstrate a compact diode-laser-pumped quantum light source for the generation of quantum correlated twin beams with a maximum squeezing of 6.5 dB.The squeezing is observed at a Fourier frequency in the audio band down to 0.7 kHz which,to the best of our knowledge,is the first observation of sub-kilohertz intensity-difference squeezing in an atomic system so far.?.The orbital angular momentum continuous-variable entanglement with the degree of 4 dB is experimentally generated using the four-wave mixing process without a cavity.