On Spontaneus Emission Of Broadening Atoms And Quantum Entangled States In Cavity QED

Cavity quantum clectrodynamics(QED) is one of the important systems of the traditional quantum optics. The rich contents for physics in this system have been attracted intensive attentions. As the physical system of quantum information processing, the main obstacles for the implementation of quantum information in cavity QED is the decoherence that is caused by the interaction with environment (finite temperature) and leaking of cavity in the beginning of this study. However, two progresses had been achieved for this topic recently. One is the cavity QED with large detuning, in which the system is insensitive to the environment and docs not require the transfer of energy between atom and field so that the time of decoherence is much increased. The another is that the decoherence processes can be seemed as a positive effect for quantum information via devising ingenious schemes. The two results make cavity QED be the one of the most prospect scheme in quantum information technology.In this paper, we study the spontaneous emission of broadening atoms in micro-cavity and quantum information processes in cavity with large detuning. The main content is as follows:1. The spontaneous emission of broadening atoms or molecules on the surface of a micro-dropletA sphere with size of some microns is easily made in experiment not only by liquid but also by solid and acts as a cavity. In this cavity many optical phenomena, such as fluorescence, lasing, and many nonlinear optical processes, have been intensively studied and a great progress has also been achieved. However the experimental result of spontaneous emission for dye molecule on the surface of a droplet cannot be explained for long time. We study the problem in detail in this paper.The density of states for the micro dielectric sphere is derived from quasi-normal models (QNM's) expansion of the correlation functions of electromagnetic fields. In the full quantum theory framework, the basic equation of the interaction between a. two-level atom and cavity fields is evaluated. The explicit expression of the spontaneous emission decay rate for a surfactant broadening molecule on the surface of a micro-droplet with radius a is obtained , in which only 1/a and 1/a~2 components exhibit. Then we apply this expression to a real experiment and obtain a consistent result with the experiment. In this process local field correction and the freedom of electric dipole of molecule have to be. considered. Meanwhile, the local field correction and the broadening of molecule can be measured precisely and easily by these fluorescence; experiments.2. The transfer of quantum information between atom and cavity field in cavity QED with large detuning and the nonclassical effects of entangled states for continuous variablesJancys-Cummings model with large detuning is the one of important candidates to implement quantum information technology. We study the transfer of quantum information between atom and cavity field in this system. On the basis of the past studies, the transfer of quantum information between atom and cavity field can be realized by exert a pulse of classical field on the atom after the atom is passed the cavity. Especially, when the information of atoms is transferred to cavity fields, the entangled state for continuous variables is generated by this means and the scheme can be extended directly to the case of multi-atom and multi-cavity.The systems with continuous variables in quantum information have more advantages beyond comparing with discrete variables systems for quantum communication or for quantum computation. Such as no single photon source and no counting of single photon arc needed. Only is the homodyne measurements needed and so that the success probabilities and efficient are all high. We can study this kind of quantum information processing by means of the technology of traditional quantum optics. Based on total variance of a pair of Einstein-Podolsky-Rosen (EPR) type operators, we introduce a sim- ple criteria of entanglement. According to the idea of concurrence introduced by Wotters we also define the degree of such entanglement. Through analyzing some specific cases, we see that this method is very convenient and easy in practical application. Especially we prove that the generalized EPR entanglement must be the two-mode squeezed state. But entangled coherent states under some conditions exhibit no any such squeezing, which reveals that there certainly exist some relations with other nonclassical properties of fields. Through studying the nonclassical properties of some entangled coherent states, we find that entanglement is closely related to nonclassical properties and the relation is exact corresponding or accompany for some states.3. On quantum information processes in cavity QED driven by classical intense field with large detuningBoth the theory and experiment on quantum teleportation are the motive force of quantum information. Quantum entangled states are the basic source in quantum information processing. How to produce and storage the entangled states is the key in quantum information technology. We study the cavity QED with large detuning and driven by classical intense field. The system is insensitive to the loss of cavity and influence of environment and is an ideal system for quantum information processing.We propose a scheme to realize the controlled teleportation of any unknown two-atom state by cavity QED driven by a classical field. In this scheme there are two cavities, one is for sender and receiver, the other is for controller. By the Hadmard operation of the controller we can complete the teleportation of quantum information. This scheme does not involve the Bell-state measurement directly. Meanwhile this approach can be used to teleport the unknown multipartite GHZ state. The probability of the success in our scheme can reach 1.0. On the other hand, the scheme can be generalized to have more controllers and is convenient to build quantum information net. When there are multi-atom in the cavity the effective Hamiltonian is same as that of Ising model. So the system can simulate Ising model. That the coupling parameter can be regulated artificially will deepen the study of this model for statistical physics. The result can give a reference of looking for a good system of quantum information in solid physics.When four maximum entangled states (Bell states) for two two-level atoms enter this cavity, the maximum entanglement does not change at any time whether the time of interaction is long or short. The system can make the initial four Bell states of two atoms be at maximum entanglement all the time. In these four states, two Bell states are just the eigenstates of the system, for the other two Bell states one of the two atoms is only operated locally and the maximum entanglement is also preserved. So this system is the simple memory for maximal entangled states.Genuine multi-party entangled state that is constructed by quantum teleportation recently is a new multi-party entangled state. The idea is the new progress in the study of entangled state. Unfortunately this method can only construct an even multi-party genuine entangled state. We construct an odd multi-party genuine entangled state to perform controlled teleportation of an arbitrary N-qubit state. We take the generalized global entanglement and the associated auxiliary measures as a standard, comparing this state with the well-known GHZ and W states. The result shows that the entanglement of this state is greater than those of GHZ state and W state.When we make two maximal entangled atoms (AB and CD) be reorganized, the pair of atoms A and C and the other pair of atoms B and D enter the different cavity respectively. Choosing the proper time of interaction, the system can easily produce four parties genuine entangled state. This is the first protocol to realize genuine multiparty entangled state in physical system and is a choice of exploring the multi-party entangled state in experiment. The above results will accelerate the research of quantum information processing with multi-party.