Theoretical And Experimental Investigation For Quantum Entanglement Swapping And Quantum Key Distribution With Continuous Variables

As well known, the phenomenon of entanglement is one of the quintessential features in quantum mechanics. It has been recognized that quantum entanglement is an important resource in quantum information and computation. Due to utilizing quantum entanglement on classical physics, can be accomplished. Such as, using entanglement shared by sender and receiver and local operation as well as classical communication, various features of unknown quantum states can be teleported from one place to another remote place with high-fidelity. In the other side, quantum dense coding can improve the channel capacity of classical signals to the extent breaking through the limit of classical communication. Further, the quantum entanglement swapping that entangles two quantum systems that have never interacted with each other can be also completed. Recent years, the multipartite entanglements have been used in quantum communication network. For example, the controlled dense coding and quantum state sharing have been experimentally realized. Researchers have also extensively applied the fundamental properties of quantum entanglement to cryptography. A variety of protocols of quantum key distribution depending upon entanglement exhibit especial advantages. The development of quantum information must play an important role in the revolution of information industry in 21th century.According to that the eigenstates of applied quantum systems, are with discrete or continuous spectrum construction, quantum information is divided into two types of discrete variables (dv) and continuous variables (cv). They have different features and application potentials. Both of them are parallelly developing. Generally, quantum information starts fromdiscrete variables firstly, and then is extended into the field of continuous variables. In this Ph.D thesis, I am going to present the experimentally and theoretical achievements on cv entanglement swapping and quantum key distribtion which were completed during the study for my Ph.D degree. The accomplish experimental investigations on cv quantum information are based on utilizing the entanglement between amplitude and phase quadratures of quantized electromagnetic fields. Following five parts will be presented:1. Utilizing two nondegenerate optical parametric amplifiers with the same configuration operating below oscillation thresholds, a pair of independent EPR entangled states with classical coherence were obtained. The correlation variances of produced EPR beams are 4.1dB and 4.3dB below their shot noise limits, respectively. Accounting for the effect of the electronic noise the real quantum correlations should be 4.9dB and 5.1dB below the SNL, respectively.2. Using two sets of EPR beams and the direct measurement of the Bell-state, the unconditional entanglement swapping for continuous variables is experimentally demonstrated. The quantum correlation degrees of 1.23dB and 1.12dB below the shot noise limit for the amplitude and phase quadratures are measured straightly between two modes which never directly interact after the entanglement swapping is achieved3. Utilizing nondegenerate optical parametric oscillator working above oscillation threshold, twin beams with the intensity correlation of 5.0dB were experimentally obtained.4. With twin beams with intensity correlation, the quantum key distribution is experimentally demonstrated by means of the directly local measurements to the intensity quantum noises of respective signal and idler modes of twin beams. The noncloning of quantum systems and the...