A quasi-transparent propagation phenomenon is presented for a pair of frequency-modulated laser pulses in an optically thick medium of cold atomic gas. The noninteracting, identical lambda-structured atoms are driven by two classical laser fields, which are frequency-chirped in the same way around the corresponding atomic transition frequencies, maintaining two-photon (Raman) resonance. It is shown by numerical analysis that after propagating over a relatively short distance (determined by the absorption length), the frequency-chirped pulse pair is affected by the atoms in such a way that instead of exciting the atoms (as it would happen in an optically dilute medium), they create a certain coherent superposition of the ground states (which can be varied by the parameters of the incoming fields), and they propagate in the remainder of the medium without significant further losses. This quasi-lossless propagation effect of the Raman-resonant frequency-chirped pulses, described above, is not only interesting in the point of view of the laser fields, but the on-demand creation of coherent superpositions among atomic states along the optically thick medium may also find applications in quantum optical experiments and quantum informatics.
The schemes of storing of images in quantum states of atoms being used nowadays are based on electromagnetically induced transparency. The images are stored in the collective atomic coherence with the storage time limited by different relaxation processes in the system with the transverse relaxation being the most detrimental among them. In this communication, we present a method of coherent writing of optical information (a transverse image) into the populations instead of the coherences of the metastable atomic states. The method is based on an action of a sequence of frequency chirped laser pulses on an atom with lambda-structure of working levels. Such storage results in drastic increase of the storage time. The reading out of the stored information is performed by measuring the population of one of the metastable atomic states.
We propose and analyze a scheme for creation of coherent superposition of meta-stable states in a multilevel
atom. The scheme is based on interaction of a frequency modulated (chirped) laser pulse and a pulse of a
constant carrier frequency with the atom having two meta-stable (ground) states and multiple excited states. The
negligible excitation of the atoms is a priority in the proposed scheme to eliminate the de-coherence processes
caused by the decay of the excited states. The scheme is applied to create coherent superposition of magnetic
sublevels of ground states of the 87Rb atom taking into account all allowed electric-dipole transitions between
magnetic sublevels of the 5 2S1/2
- 52P3/2 transition (D2 line).
In addition to the theoretical analysis we consider possible experimental realizations of the proposed
coherence creation scheme and discuss their feasibilities and constraints. We concentrate on a detection of the
superposition state in the Faraday-rotation experiment. Such detection reduces technical laser noise background
and offers high sensitivity of the coherence detection. Moreover, it allows extra control of the atomic sample and
the interaction dynamics by external magnetic field.