Quantum Coherent Effects And Their Control In Impurity-doped Bose-Einstein Condensate Systems

An impurity-doped Bose-Einstein condensate(BEC)system is an ideal platform to study quantum effects of micro-macro quantum systems.This thesis focuses on studying quantum coherence and quantum coherent effects of micro-macro quantum systems.We show how to generate and control macroscopic quantum coherence and quantum coherent effects such as micromicro entanglement,micro-macro entanglement,and macroscopic quantum superposition states.The main results of the thesis are included as follows.1.We study quantum dynamic properties of the single impurity-doped BEC.We introduce the concept of the single-atom valve(SAV).It is shown that the impurity atom immersed in the two-component BEC can act as the SAV to control quantum coherence of the two-component BEC.It is demonstrated that the SAV can realize the on-demand control over quantum coherence at an arbitrary time.Specially,it is found that the SAV can also control higherorder quantum coherence of two-component BEC.We investigate the long-time evolution of quantum coherence of the two-component BEC,and find the imuprityinduced collapse and revival phenomenon,which reveals the quantum nature of the BEC matter-wave field.It is indicated that collapse-revival patterns of quantum coherence can be manipulated by the initial-state parameters of the impurity atom and the impurity-BEC interaction strengths.2.We study complementarity between micro-micro and micro-macro entanglement in a Bose-Einstein condensate with two Rydberg impurities.We investigate quantum dynamics of micro-micro and micro-macro entanglement in the micro-macro system.It is found that strong micro-macro entanglement between Rydberg impurities and the BEC can be generated by the use of initial micro-micro entanglement between two Rydberg impurities,which acts as the seed entanglement to create micro-macro entanglement.We demonstrate a curious complementarity relation between micro-micro and micro-macro entanglement,and find that the complementarity property can be sustained to some extend even though in the presence of the BEC decoherence.3.We study generation and manipulation of macroscopic quantum superposition states(MQSS)of the BEC and the influence of BEC decoherence.We propose two schemes to create MQSS of the BEC.One is the probabilitycreating scheme by means of project measurements,another is a deterministicallycreating scheme.It is shown that the structure and properties of the MQSS can be manipulated by changing the number and parity of the impurities.It is indicated that decoherence of the BEC degrades the quality of the created MQSS.It is found that one can prepare fastly MQSS and enhance the fidelity of the MQSS by means of increasing the impurity-BEC coupling strength or the number of impurity atoms.