Controlling Of Quantum Entanglement In Various Environmental Models

Quantum information science is the combination of quantum mechanics and informationscience. As a new cross subject, the birth and development of quantum information science is offar-reaching significance in science and technology. Compared with classical informationprocessing, the quantum information processing has great advantages. For example, a quantumcomputer can exponentially speedup the algorithms that cannot be performed with a classicalcomputer, and quantum communication enables us to transfer information in a definitely safemanner. As everyone knows, the necessary requirement for performing quantum informationprocessing is that the system is quantum-correlated. Quantum entanglement is an importantresource in quantum information science, and its development determines whether the quantuminformation science has a widen application foreground. However the inevitable interactionbetween the quantum system and environment will eventually destroy the quantum entanglementof the whole quantum system. This local decoherence is the main obstacle in the realization ofthe quantum computer. So the study of entanglement dynamics of open quantum system undervarious environmental models is necessary and meaningful. The main results of this thesis are asfollows:1. Considering three different environmental models (i.e., single reservoir model, commonreservoir model and independent reservoir model), we investigate the evolutional characters ofthe entangled system initially structured in Bell-like states by utilizing the pseudomode method.Through comparing the entanglement decays under these three different environmental models,we acquire an optimal environment model in which the initial entanglement would be preservedmuch better. In particular, when the spectral density of reservoir is satisfied as an ideal photonicband gap, we find that the initial entanglement between two qubits via these three differentdecoherence models all would display entanglement trapping. By comparing the roles ofdifferent decoherence models on the entanglement trapping for the same initial entangled state,we examine the conditions in which the larger entanglement trapping can be achieved.2. We investigate the roles of different environmental models on quantum correlation decayof a two-qubit composite system interacting with two independent environments. The mostcommon environmental models (the single-Lorentzian model, the squared-Lorentzian model, thetwo-Lorentzian model and band gap model) are analyzed. We find that, for the weak couplingregime, the two-qubit quantum discord in the single-Lorentzian (band gap) environment is morerobust than in the squared-Lorentzian (two-Lorentzian) environment under the resonant and nearresonant conditions, while for the far off-resonant condition the two-qubit quantum discord in thesingle-Lorentzian (band gap) environment decreases much faster than in the squared-Lorentzian (two-Lorentzian) environment. However, by considering the strong coupling regime we find thatthe two-qubit quantum discord is more robust in the squared-Lorentzian (two-Lorentzian)environment than in the single-Lorentzian (band gap) environment, either under the nearresonant or the far off-resonant condition. We give out the reasonable explanations from spectraldensity function and environmental non-Markovian effect for the different results of these tworegimes. We note that, for the weak coupling regime, the monotonic decay speed of the quantumcorrelation is mainly determined by the spectral density functions of these different environments.While, by considering the strong coupling regime, contrary to what is stated in the weakcoupling regime, we find that the dynamics of quantum correlation primarily depends on thenon-Markovianity of the environmental models.3. We propose a scheme to enhance trapping of entanglement of two qubits, in thecommon environment of a photonic band gap and two independent environments of photonicband gaps respectively, by weak measurement and quantum measurement reversal. Firstly,before the qubits undergoing decoherence, we perform a weak measurement on each qubitsrespectively. While after the system undergoing the evolution process, a suitablepost-measurement (weak measurement or quantum measurement reversal) on these two qubitsare performed. In the common photonic band gap model, we study the relation among theoptimal entanglement trapping, the corresponding success probability and weak measurementstrength. Moreover, we indicate that the prior weak measurement can be used to prevententanglement sudden death (ESD) in the photonic band gap, but the post measurements alonecannot circumvent ESD-causing. In the independent photonic band gap models, we mainlyinvestigate the relation among the optimal entanglement trapping, the pre-measurement strengthand the system initial states.4. We study the physical mechanism on generating the quantum entanglement and quantumdiscord via the initial classical-mixed states. Using the pseudomode method, we theoreticallyanalyze a realistic situation in which two two-level dipole-dipole interacting atoms coupled witha common structured reservoir with different coupling strengths. By considering certain classesof initial separable-mixed states, we find that the atomic entanglement sudden birth takes placeand the generation of stationary quantum correlations occur. In addition, the occurrence time ofentanglement sudden birth and the stationary values of quantum correlations are related to theinitial conditions of states, the dipole-dipole interaction and the relative coupling strength.