Resonant Gain Suppression And Quantum Destructive Interference In A Three-level Open V System

Electromagnetically induced transparency (EIT), referring to the resonantabsorption reduction in a narrow spectral region, is a well-known phenomenon basedon quantum destructive interference and has attracted great attention due to itsimportant applications in many aspects of quantum optics. For instance, with the EITtechnique, one can dramatically reduce the group velocity of a weak light pulse to afew meters per second and even to zero, which can make the weak light pulse store inthe medium. One remarkable advantage of this light storage technique is thatamplitudes, phases, and polarizations of light pulses change little during storage,which is of great significance for optical quantum information processing. EIT is alsoa main driving force for achieving other interesting phenomena such as lasing withoutpopulation inversion (LWI), enhancement of optical nonlinearities and fastmagneto-optical switching. In this paper, first of all in the second chapter they discuss the regulation of thegroup velocity of light pulses in various models (lambda, ladder, and V-type) quantum interference nature of the atomic system, Second, in the third chapter weinvestigate the steady optical response of a four-level atomic system, which may beregarded as a three level open V system because one level is coherently decoupledfrom the other levels, driven by a weak coherent field (probe), a strong coherent field(coupling) and a weak incoherent field (pump). Both (approximate) analytical and(full) numerical calculations will be performed for real and imaginary parts of the probe susceptibility based on the optical Bloch equations. Our results show that anarrow and deep transparency window accompanied by steep abnormal dispersionexists between two gain lines on the probe resonance, which is in fact a signature ofperfect quantum destructive interference. The existence of such quantum interferenceis well analysed by three different methods in the dressed state representation of thecoupling field. First, we replace the bare-state levels with dressed-state levels and tryto understand the perfect quantum destructive interference in terms of dressed-stateatomic transitions. Alternatively, we can understand the perfect quantum destructiveinterference by splitting the imaginary part of the probe susceptibility χ into several terms of different physical meanings in the dressed state representation of thecoupling fieldω c. Finally, we understand the perfect quantum destructive interferenceby comparing steady populations in the open V system with those in the closed Vsystem.It is found that, to attain perfect quantum destructive interference between twodressed-state transition pathways, the lower level in the open V system should have aspontaneous decay rate much larger than those of the two upper levels. This specificsituation may be realized when both upper levels in the open V system refer to highlyexcited Rydberg states of radiative lifetimes up to tens of microseconds. We alsoshow a light pulse ultrafast propagating in a cold sample of87Rb atoms driven into theopen V configuration. Due to a narrow and deep transparency window accompaniedby steep abnormal dispersion between two gain lines on the probe resonance in thesystem, the group velocityυ gof the light pulse on the resonance is always largerthan the light speed c in vacuum and can even achieve a negative value. It is worthnoting that, in the open V system, the light pulse will not experience remarkable gainor loss while it propagates with an ultrafast velocity. But, in other schemes forattaining ultrafast light propagation the light pulses may experience large gain or lossso that they have severe deformation.