Generation Of Quadripartite Entangled Optical Field And Quantum Key Distribution With Continuous Variables

Quantum information is an advanced research topic in modern science and technology. The fresh subject devotes to exploit the fundamental principle of quantum mechanics and the amazing characteristic of quantum states, such as quantum entanglement and superposition of state etc, to perform the processing, computation and transmission of information. 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. Utilizing quantum entanglement, we can accomplish the impossible tasks in the frame of classical information and computation. Utilizing quantum entanglement, we can implement the disembodied transport of an unknown quantum state from one place to another remote place with high fidelity, i.e. quantum teleportation. Utilizing quantum entanglement, we can also improve the channel capacity of classical signal transmission by means of quantum channel, i.e. quantum dense coding, even we can entangle two quantum systems that have never directly interacted with each other through entanglement swapping. The fundamental properties of quantum physics have been applied to cryptography. Quantum cryptography based on quantum mechanics can ensure genuine security of communication in principle and realize the absolutely secure quantum key distribution (QKD). A variety of quantum key distribution protocols depending on quantum entanglement exhibit especial advantages. Designing and experimentally realizing quantum key distribution have not only the significance for the fundamental research but also the potential application requirement.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 developing in parallel. Generally, quantum information starts from discrete variables originally, and then is extended into the field of continuous variables. The cv quantum communication has attracted wide interests due to its potential advantages such as high bit transmission rates. However, with the comparison to the dv system, the experimental investigation of cv quantum communication is relatively lagging. So far, the fundamental experiments, such as unconditional quantum teleportation, quantum dense coding and quantum entanglement swapping, have been realized with cv Einstein-Podolsky-Rosen (EPR) entanglement. However, the key for developing quantum information is to realize quantum information network. For developing quantum information network a most important step is to generate multipartite entangled states experimentally.The main research contents of the thesis are as followings:1. We designed the generation system of cv quadripartite GHZ-like and Cluster-like entangled states and accomplished the experiment generating quadripartite GHZ-like and Cluster-like entangled states. We theoretically and experimentally demonstrated that the two different types of quadripartite entangled states can be obtained by the linearly optical transformation of four squeezed states (two amplitude-quadrature squeezed states and two phase-quadrature squeezed states) produced from a pair of nondegenerate optical parametric amplifiers operating at deamplification under appropriate phase relations. (Phys. Rev. Lett. 98, 070502 (2007); Science in China, accepted)2. We proposed a scheme of continuous-variable quantum key distribution, in which the bright EPR entangled optical beams are utilized. The source of the entangled beams is placed inside the receiving station, then a half of the entangled beams is transmitted with round trip between the receiver and the sender and the other half are retained by the receiver which is never opened. Two sets of signals respectively modulated on the amplitude and phase quadratures of the signal beam by the sender are simultaneously extracted by the authorized receiver with the scheme of the dense-coding correlation measurement for continuous quantum variables, thus the channel capacity is significantly improved. The proposed scheme can beat the 3 dB loss limit of the cv quantum key distribution when the high quantum entanglement is utilized. (Phys. Rev. A 74, 062305 (2006))3. We implemented the cv quantum key distribution using the EPR correlation of the optical field directly. Two communication parties share a pair of bright EPR entangled beam and measure the amplitude or phase quadratures of their own optical fields randomly. Then, the secret key is established by means of quantum correlation between their amplitude and phase quadratures. In the proposed QKD scheme without the signal modulation the secret key is established and the eavesdropper is discovered through the local measurement of quantum fluctuations and the public comparing of the measured data. The security of the scheme was proved with information theory. (Paper in preparing)4. The frequency-nondegenerate entangled beams with total intensity of 22 mW were produced from a nondegenerate optical parametric oscillator operating above threshold. The correlation degree of amplitude-quadrature and phase-quadrature measured by unbalance Mach-Zehnder interferometer are 1.25 dB and 0.60 dB, respectively. The experimental design provides an effective approach to prepare and detect the frequency tunable entangled optical field high intensity.The completed creative works are as follows:1. We designed the experimental generation system of cv quadripartite GHZ-like and Cluster-like entangled states, deduced the fully inseparability criteria of quadripartite Cluster-like entangled states theoretically and calculated the dependences of quadripartite entanglement on experimental parameters numerically. We experimentally obtained the two kinds of cv quadripartite entangled optical fields for the first time. 2. We proposed a scheme of continuous-variable quantum key distribution utilizing the bright Einstein-Podolsky-Rosen entangled optical beams and roundtrip transmission. The possibility of beating 3 dB loss limit with higher entanglement was theoretically proved.3. We proposed a scheme of continuous variable quantum key distribution without signal modulation using EPR correlation directly and experimentally proved the feasibility of the scheme.4. We prepared the frequency-nondegenerate entangled optical field with the total intensity of 22 mW by means of nondegenerate optical parametric oscillator operating above threshold and measured the entanglement degree with a pair of unbalanced Mach-Zehnder interferometers.