Studies Of Atomic Coherence Effects

Quantum coherence and interference has become one of the important forward subjects in laser physics and quantum optics. Recent studies have shown that the quantum coherence and interference have led to a lot of new effects such as coherent population trapping, electromagnetically induced transparency, index enhancement without absorption, etc. These new effects have great potential for the control of the coherence properties of a medium, the production of high frequency lasers, the information storage and high precision measurements.In this paper we have predicted two kinds of new quantum coherence effects. One is the strong-field index enhancement via selective population trapping of dressed states and the other is the preservation of the coherent population trapping (CPT) is by quantum interference of incoherent pump processes.First, we consider strong field index enhancement. The significant feature of the system model is that the two-dipole moment is in the V configuration. Of the two dipole allowed transitions, one is very strong while the other is very weak. The strong probe field is coupled to the strong transition and creates two dressed states, the level spacing of which is far larger than the atomic decay rates. The weak control field is coupled with the weak transition, which is separated into two transitions from an auxiliary into either of the two dressed states in the dressed states representation. When the control field is tuned resonant with one dressed transition, the other dressed transition is negligibly small. The resonant dressed transition and the spontaneous decay of the other transition constitute a two-step pathway of population transfer from one dressed state to the other. As the consequence, the population is trapped in one dressed state. This kind of coherent trapping leads maximal atomic coherence, which characterizes ultralarge index of refraction and vanishing absorption. A typical model is the atomic barium in which the state 6s 1S0 corresponds to the ground state and the states 6s6p 1P1 and 6s6p 3P1 correspond to the excited states of strong and weak transition, respectively. Thedecay rates of the excited states are 47kHz and 19MHz, respectively, and the ratio of them is 400. The conditions are reached when the intensity of the probe and control fields take 177W/cm2and 4.45mW/cm2. It is clear that such conditions for the realization of the present scheme are accessible to the experiment.Next we consider the preservation of CPT by quantum interference of incoherent pump processes. It has been shown that at the heart of various quantum coherence effects lies CPT. One typical model for CPT is the A type three-level system when the life times of the ground levels are long enough and the coherent driven transitions are on the two-photon resonance. It has also been found that the incoherent pump process generates quantum interference. Quantum interference occurs in a three-level A system with near-degenerate dipole allowed transitions, which are coupled to the same linearly polarized incoherent field. It is generally believed that the incoherent pump destroys CPT. In this paper we have shown that this is not always true. The quantum interference of incoherent pump preserves CPT. When the polarization direction of the incoherent field and the amplitude and phase of the applied coherent fields satisfy certain conditions, the coherent trapping effect is maintained. We have separated the dark state and bright state, which are both the superposition states of the ground states. Between the dark and bright states there is a unidirectional pathway of population transfer from the bright to dark state. As a consequence, the population transfer leads to CPT.