Study On The Effects Of The Quantum Interference In The Ladder Four-level-system Driven By Two Coherent Fields

Quantum interference has attracted more and more interestsin recent years. The interaction of an excited atom with the vacuum results in the spontaneous emission of the radiation. This radiation has an isotropic distribution in space which leads to a Lorentzian profile and its linewidth in the frequency domain are proportion to the Einstein spontaneous decay rate. It is responsible for many important physical phenomena such as quantum noise of an optoelectronic device and the limited laser linewidth and so forth. For a long time, people have desired to find an approach by which spontaneous emission rate of an excited atom can be depressed or cancelled. Since the spontaneous emission are directly related the electronic dipole moment and the environment where the atom is placed, therefore, there are two ways to modify the spontaneous emission of an atom. By placing an excited atom in a confined environment such as a photonic crystal and a resonance cavity, on the one hand, spontaneous emission can be controlled by tailoring the density of the electromagnetic modes of the environment in the neighborhood of the resonant frequency. On the other hand, the cancellation of spontaneous emission of an excited atom can be obtained via quantum interference between different atomic spontaneous decay pathways. Furthermore, this method has been attracting a lot of attention.In this paper, we take the Ladder-configration of a four-level atom into account. We calculated numerically several important phenomena result from the quantum interference such as ultra-sharp spontaneous spectra and absorption spectra, EIT, CPT etc. This paper is organized as follows. Part one is an introduction. In part two, the basic theory we used has been given. In part three, the model is presented and the formula for the numerical calculation are developed. And then we discuss the physical phenomena caused by quantum interference as follows. The conclusions are summarized in part five.inThrough our numerical calculation, we obtained several useful results in our model. First, we investigate the ultra-sharp EIT and find that the Rabi frequency of upper coherent field does not affect the occurrence CRT but the width of the CRT, then discuss the modification of absorption and dispersion profile, after that we study the effects of two pairs of pathways on the spontaneous emission spectra and the absorption spectra. Finally, we take the laser width and Doppler effect into account and find that the laser width destroys the quantum interference effect.