Atomic Coherence Effect And Entangled Light Generation

Quantum entanglement shows that there exist the nonlocal and nonclassical quantum correlations between two or more quantum systems,which is one of most striking features in quantum mechanics.Recently,the preparation of quantum entanglement and its applica tions have been the frontiers of quantum information science.Quantum entanglement not only can be used to test the fundamentals of quantum mechanics,such as realization,local-ization, hidden variable,measurement theory and so on,but also has practical applications in quantum information processing,for example,quantum teleportation,quantum dense coding and quantum key distribution.On the other hand,atomic coherence effect is an important topic in quantum optics and laser physics and it always has been one of hotspots in scientific research.Atomic coherence effect leads to many interesting physical phenom ena,such as coherent population trapping,electromagnetically induced transparency and enhanced nonlinear optical processes,which have the certain values in the process of basic physics and practical applications.Our topic is to prepare the steady noise-free entangle-ment sources with a high purity based on atomic coherence effect.The innovative contents involve the following three aspects:We first analyze quantum entanglement in four-wave mixing in three-level∧and V atomic systems,in which two external driving fields are applied to the dipole-allowed tran sitions and two cavity fields are generated,respectively.We analyze the physics by using dressed atomic states and squeeze transformed cavity modes.In such a representation,two dissipation channels are identified,through which dressed atoms simultaneously absorb in the excitations from the pair of squeeze transformed modes.It is in the presence of two channels that the entanglement is greatly enhanced.With the proper choice of the normal ized detuning,the best achievable state approaches the original Einstein-Podolsky-Rosen (EPR) entangled state.This scheme is applicable in the optical regime where atomic spon taneous emission has to be taken into account.We then show that EPR light entanglement is obtainable in the microwave controlled four wave mixing,where the control and generated fields differ in frequency by five orders of magnitude.In particular,an ensemble of three-level∧atoms is employed,in which a microwave field resonantly couples two metastable states and a laser field resonantly couples one of the metastable states to the excited state.A pair of inner sidebands as optical cavity modes are amplified via the transition from the excited state to the other of the metastable states.The analysis is presented by using the dressed-atom squeezed transformed-mode approach.For a proper ratio of the amplitudes of the microwave and the applied laser field,the sum of the variances for two EPR-like operators approaches zero, which corresponds to the EPR entanglement.Finally,we propose an alternative scheme for preparing N-qubit cluster state by us ing a frequency-modulated laser field to simultaneously illuminate the hot ions,which are trapped in a linear Paul trap.Selecting the index of modulation yields the selective mecha-nisms of coupling and decoupling between the internal and external states of the ions.Based on the selective mechanisms,the highly entangled cluster state is achieved.In our scheme, the vibration mode is only virtually excited.Thus the quantum operations are insensitive to the heating and lead to the high-fidelity quantum information processing.