Coherence-induced entanglement

Coherence and entanglement are the two key concepts that distinguish quantum mechanics from classical mechanics. Many novel phenomena occuring in the quantum world are due to these two ??physical quantities??. They also play essential roles in quantum computation and quantum information. For example, coherence, which says that a quantum mechanical system could be in a superposition state, makes the quantum parallel computing scheme possible; and entanglement, which says that two quantum systems separated in space could be in an intervened state, is the key factor in various quantum teleportation algorithms. We have studied entanglement generation in various systems. We found that with atomic coherence, entanglement could be generated between two thermal fields with arbitrarily high temperatures. We also found that temperature difference instead of the purity of state is essential for the entanglement generation between an atom and a thermal field. We discovered that correlated spontaneous emission lasers (CELs) could be used to generate bright entanglement laser beams. As a special case of CEL systems, we studied entanglement generation in Non-degenerate Optical Parametric Amplifiers (NOPAs). We performed the input-output calculations for a NOPA system and showed that the two output optical beams are still entangled. This justifies our idea that CEL (or NOPA) systems can be used as an ideal entanglement source for various quantum information schemes. From an experimental point of view, we considered the effects of pumping fluctuations on entanglement generation in CEL and NOPA systems. We found that these fluctuations, especially the phase diffusion processes, in the pump laser would greatly reduce the entanglement generated in such systems.