Preparation Of Quantum States And Dense Coding In Linear Optical System

Since the early1990s, Quantum information gradually developed into an emerging hot spot disciplines, attracting a growing number of scientific and technological workers to join the research team. There are two branches, quantum computation and quantum communication, Their core objective are to create a true practical application of quantum computers and realize absolutely safe and practical quantum communication of long distance. Quantum computer is still in the phase of theoretical research, and distance quantum communication research has begun to gradually move from theory to experiment.Quantum communication is to deliver and process information based on quantum methods, its main contents are quantum teleportation (achieving the remote transmission of unknown quantum states while not sending any particles), quantum dense coding (transmitting two bits of information by sending a qubit). The process of quantum information processing in the final analysis is the process of preparation and operation of quantum entangled states.Currently, people have tried to study quantum information in a variety of systems, common physical systems have linear optics, cavity QED, the ion trap, nuclear magnetic resonance, quantum dots, low-temperature superconducting, etc., the difference of these systems is based on different information storage ways.The optical system is a good quantum information processing system, and the experimental development of the optical system is very quickly. This article focuses on preparation of multi-body entangled state and implementation process of quantum dense coding in the optical system.1. Preparation of the GHZ states in an optical system.We propose a scheme for preparation of polarized three-photon GHZ state in linear optics, combined with nonlinear cross-Kerr media and the use of homodyne detection, and this program can be extended for the preparation of N photon GHZ state, and compared to other schemes, the advantages of this scheme is a high probability of preparation, almost close to100%.2. Four-particle states of quantum dense coding in an optical system.In the linear optics, we use linear optical devices, proposing a four-particle entangled state dense coding scheme. Throughout the program, we use quantum non-demolition detector, this kind of detector also uses homodyne detection, the use of interaction by the signal mode and the detection mode, in the nonlinear cross-Kerr medium, the probe light phase-change measurements. The success probability of our dense coding scheme is close to1.These two schemes are belong to the linear optics, the photon is a neutral particle, it is relatively long decoherence time. Therefore, the external environment influence is relatively small, less likely to destroy the photon.Used in the above scenario the nonlinear cross-Kerr effect can be achieved in the laboratory, as long as the intensity of the probe light field is large enough, then the requirements of cross-Kerr effect intensity will not be great. The use of electromagnetically induced transparency, in the laboratory can obtain this strength of the nonlinear cross-Kerr effect intensity. Homodyne detection not only can avoid the single-photon detection destruction of the photon, and can also be feasible experimentally.Linear optical components used in our program, such as PBS, BS, the FS-PBS, etc., are commonly used in quantum optics experiments. It is easier to realize a variety of single-bit operations on the photon, so the experimental feasibility is very high.