Concentration And Protection Of Entanglement

Last two decades witnessed a fast development of quantum information. Quantum information science consists of quantum communication and quantum computation. Quantum information is widely noted for its unique characteristics. In quantum information, quantum entangled state plays an important role. It is used in quantum teleportation, quantum encoding, quantum key-distribution, romote entanglement concentration, the test of quantum nonlocality. The research on quantum entanglement can help us to understand quantum mechanics better. The fidelity and efficiency of quantum communication are both determined by the entanglement degree of quantum entangled state serving as quantum channel. Only when the quantum entangled state we used is a maximally entangled state, the fidelity and efficiency of quantum communication are both1.0. However, there exist interactions between a quantum system and its surrounding environment inevitably. Meanwhile quantum operations will bring in some errors. So the quantum entangled states we can achieve in experiments are all non-maximally entangled states. They will reduce the fidelity and efficiency of quantum communication and weaken quantum communication’s capacity. So entanglement improvement and entanglement protection are needed.In this thesis, we focused on the following two works:(i) Entanglement concentration. We will apply generalized measurements to entanglement concentration of known non-maximally entangled pure states in arbitrary dimensional system. Here, how to design the generalized measurements for the unambiguous discrimination of linearly independent non-orthogonal states is crucial for the concentration of the known non-maximally entangled states. A physical scheme is designed for the unambiguous discrimination of linearly independent non-orthogonal states in cavity QED system. The result shows that, any known non-maximally entangled pure state (for arbitrary dimensional system) can be transformed to the maximally entangled state only by introducing a qubit as ancilla and a joint unitary transformation operation on one of the entangled particles and the ancilla. In addition, because the less entangled state of each failed round will be re-concentrated too, the entanglement waste during the concentration process will be greatly reduced.(ⅱ) Entanglement protection. For the atoms in the continuum vacuum, the interactions between the atoms and the surrounding environment will lead to spontaneous emission and the entanglement will drop very fast. We will apply2π pulses sequences on the encoding level and an auxiliary level to protect the entangled atomic state against spontaneous emission. We focused on two-atom entangled states and three-atom entangled states in different-types of continuum vacuum. By computation and simulation, we find2π pulses sequences can protect atomic entangled states against spontaneous emission effectively.