Construction Of All-optical And Multiplexing Quantum Communication Protocols And Enhancement Of Quantum Correlation

In quantum information science,quantum resources with unique properties are utilized to improve the communication security,channel capacity,calculation speed,measurement precision,and to exceed the corresponding classical limits in the fields of communication,computing,and metrology.Quantum information science has been developing in many experimental systems.This thesis mainly focuses on quantum information processing in the optical system.In recent years,the four-wave mixing process based on hot atomic ensemble has been widely used in quantum information processing for the continuous-variable system with optical modes due to its strong nonlinearity,low-noise amplification property and multi-spatial-mode nature.This thesis mainly introduces the applications of the four-wave mixing process based on hot rubidium atomic ensemble in quantum information processing,including the following three experimental works:1.It is an important research topic to improve the information transmission ability of communication processes.In order to further improve the channel capacity of the quantum dense coding protocol with fixed resources in the continuous-variable system,we utilize the channel multiplexing technique and implement orbital-angularmomentum multiplexed quantum dense coding.Firstly,based on the four-wave mixing process,we generate an Einstein-Podolsky-Rosen entanglement source coded on the orbital-angular-momentum superposition mode.Then,using such multiplexed entanglement source,we implement orbital-angular-momentum multiplexed quantum dense encoding,with a channel capacity enhancement of about 2.4 d B compared to that of the conventional quantum dense encoding with the same resources.The experimental results are promising to be applied to the construction of high-capacity quantum communication networks.2.Quantum state sharing is an important quantum information protocol in quantum networks and quantum cryptography.In the continuous-variable system,the information transmission ability of the quantum state sharing protocol is limited by the electrical feedforward technique required for its experimental implementation.In order to remove the bandwidth limit of quantum state sharing caused by the optic-electro and electro-optic conversions of the electrical feedforward technique,we replace the electrical feedforward device with a low-noise amplifier based on the four-wave mixing process and experimentally implement(2,3)threshold deterministic all-optical quantum state sharing.In this protocol,the secret quantum state is encoded into three shares and then distributed to three users.We experimentally demonstrate that any two of the three users can cooperate to reconstruct the secret state,while the rest user cannot obtain the information of the secret state.We also demonstrate that quantum state sharing can be successfully implemented within a certain bandwidth range.The experimental results are expected to be applied to the construction of all-optical broadband quantum communication networks.3.In addition to the applications of the four-wave mixing process in quantum communication,quantum squeezing generated by the four-wave mixing process is an important quantum resource for quantum metrology.The measurement precision of the system can be further improved by enhancing the quantum squeezing level.We experimentally implement quantum squeezing enhancement based on phase-sensitive cascaded four-wave mixing processes.The intensity-difference squeezing generated by the phase-sensitive cascaded four-wave mixing processes is about 7.42 d B,which provides about 4.11 d B or 3.41 d B enhancement compared with the intensity-difference squeezing generated by a single four-wave mixing process(about 3.31 d B or 4.01 d B).This experimental scheme provides a new method for largely improving the intensitydifference squeezing degree and is expected to be used to further improve the measurement precision of physical quantities.