The Effect Of VIC On Velocity-selective Coherent Population Trapping

As de Broglie said, in the quantum world, all massive particles, including atoms, should be thought of as waves with certain energy and wavelength. For atoms, the lower the temperature is, the more they act like waves. Atoms under very low temperature will show some special properties (of BEC predicted by Bose and Einstein) that we can not get in room temperature. So to realize BEC, one started to become interested in cooling atoms. By now, scientists have already developed many mechanisms to get low atoms temperature. Velocity selective coherent population trapping (VSCPT) is one of these mechanisms, and it is a very important one. In this paper, we will mainly focus on introducing the basic principles of VSCPT, the mechanism of vacuum induced coherence (VIC) and the effect of VIC together with spontaneous on VSCPT.Dark state plays a very important role in VSCPT. The most interesting property of dark state is that they are immune to spontaneous and the external optical field. Usually, atoms always keep doing random walk, and by this random walk, they could fall into this dark state and remain there forever. This is just what we have called"trapped". If the momentum broaden of dark state is very narrow, it means that atoms in such dark state have very low temperature.Spontaneous is actually the result of interaction between atoms and vacuum field, i.e. the transition without the effect of external field. The transitions of atoms from different levels are usually not independent (actually, only if the electron dipole moments of these transitions are orthogonal to each other, they are independent). For these transitions, the effect from each other is called vacuum induced coherence (VIC). In this paper, we will give an explanation of VIC.The reason of atoms'random walk is spontaneous, so spontaneous plays a very important role in VSCPT. The role of spontaneous to VSCPT is just like the role of electricity to a machine. However, scientists never considered the effect of VIC when they take the positive effect of spontaneous as an important influence in VSCPT. In this paper, we will see that the results of VSCPT are usually very different with or without considering the effect of VIC. As a matter of fact, in some experiments under special conditions, VIC can generate essential difference to VSCPT.