Few-body Quantum Entanglement And Correlation Induced By Many-body System

During the past two decades, there has been profound advance in the theoreticalresearch and experimental technique of quantum information and quantum computa-tion. Ion trap, nuclear magnetic resonance, quantum dot and superconducting Joseph-son junction have been proposed as the models for quantum computation. Experimentsconcerning a few qubits were carried out for demonstration. But there still exists a cru-cial problem prohibiting quantum computers from being realized. Since a real physicalsystem always interacts with its environment, the decoherence is unpreventable. Howto reduce the noise induced by the environment lies at the heart of both theoretical andexperimental research. On the other hand, in order to put quantum computers into prac-tice, we need to integrate qubits with the number of the order of 100. Moreover, for anyunitary transformation, two-qubit operations should be realized in advance. To solvethe problems listed above, one of the feasible methods is to make use of the quantumdata bus. Physically speaking, it is equivalent to inducing entanglement and correlationbetween few bodies by simultaneous interaction with a many-body system.The thesis comes in three parts. The first part is Chapter 1. It provides a briefintroduction to the basic concepts and problems concerning quantum information. Inthis chapter, we do not only review the present status of the theoretical research andstate of the art in the models proposed for quantum computation, but also elucidatethe advantage of coupling qubits with quantum data bus. All these basic knowledgeis arranged for the following chapters. In addition, Fro¨hlich transformation and theconcept of adiabatic elimination is presented since they are frequently used thereafter.Chapter 2 is the second part. In this chapter, coupling qubits with quantum databus is reviewed in the ion trap. And the quantum storage schemes based on electromag-netically induced transparency and quasi-spin-wave inΛ-atom crystal are introduced.The third part comprises Chapter 3, 4 and 5, where concrete physical models ofquantum data bus are applied to the investigation on the quantum entanglement and cor- relation induced by them. In Chapter 3, the entanglement of two spins induced by spinensemble is well explored. In the next chapter, we discover that the coupling betweentwo external spins is enhanced by the quantum criticality. Finally, the Ising model inthe transverse field is realized with an array of capacitively coupled Cooper pair boxes.And it is discovered that photon bunching e?ect of two modes in the transmission lineis witnessed when the quantum data bus is at the critical point.