Theoretical And Experimental Investigation For Entanglement Manipulation With Continuous Variable

Quantum information is one of the advanced research topics, in which the quantum entanglement is exploied as a resource for implementing the information transmission and process and thus more secure communication and faster computation than classical schemes are able to be achieved. So far, the entangled optical fields have been applied in quantum teleportation, quantum entanglement swapping, quantum dense coding, quantum key distribution, quantum networks and quantum computation.The entangled states with possibly high entanglement degree are the essential requirement for accomplishing the quantum information transmission and process with higher fidelity. The quantum manipulation is one of the efficient methods to improve the entanglement level of entangled states of light. Multipartite entangled states are the basic resources used in quantum networks and quantum computers. Recent years, the research on continuous variable multipartite entangled states with more than two sub-systems has attracted more and more attentions. During my Ph. D study, I joined in the subject of experimental investigation for the entanglement enhancement of bipartite entangled optical fields by means of cascaded NOPAs and completed the theretical research on the multipartite entanglement enhancement using coherent feedback control. I also participated in the experimental works on the generation of the squeezed state of light at the absorption line of Rubidium atom, which can be applied in the quantum memory of a full quantum information networks.The main research contents of the thesis are:1. We experimentally realize the continuous variable EPR entanglement cascaded enhancement by using three NOPAs operated below the threshold. The first NOPA is utilized to produce initial EPR entangled optical fields, and the second and the third NOPAs are used to enhance the entanglement level of the EPR entangled states, successively. Through the cascaded system the entanglement level of the EPR entangled optical fields is increased from-5.3dB to-8.1dB below the quantum noise limit, which is the highest EPR entanglement of optical modes obtained by experiments so far to the best of our knowledges.2. We theoretically propose a multipartite entanglement enhancement system via the coherent feedback control, in which the multipartite optical entangled state consisting of entangled sub-modes with different frequencies is generated by a single NOPA and a part of the output optical field from the NOPA is fed back into the NOPA again to interfere with its intracavity field. By controlling the phase and the intensity of the fed light the quantum entanglement of the output field from the NOPA can be enhanced.3. We theoretically demonstrate that a single NOPO can produce three color entangled optical fields, whose frequecncies are at the absorption line of Rubidium atom and the transmission window of fiber, respectively.4. We experimentally obtain-2.5dB squeezed state of light with the frequency at the absorption line of Rubidium atom.The innovation works of the thesis are:1. We experimentally realize the cascaded enhancement of continuous variable EPR entanglement by using a series of NOPAs and finally obtain the optical entangled state of-8.1dB.2. We theoretically propose a multipartite entanglement enhancement scheme by means of coherent feedback control, in which the optical multipartite entangled state is produced by a single NOPA.3. We theoretically prove that a single NOPO in room temperature can generate the three-color tripartite entangled optical fields even when the photon noises exist. The frequecncies of the three entangled optical modes are at the absorption line of Rubidium atom and the transmission window of fiber, respectively.