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When several laser lights couple three or more atomic levels, quantum coherence and interference can lead to “dark resonances” or coherent population trapping (CPT). CPT is a basis for effects like electromagnetically induced transparency and absorption (EIT and EIA), large enhancement of the refractive index, atom interferometry, subrecoil laser cooling. Very narrow transparency resonances have applications in a new generation of small and compact atomic clocks. The refractive index of refraction-enhanced media has the applications in precision magnetometry. Related to the effect is the biomagnetometry, growing filed of non-invasive measurements of the magnetic fields generated by a human heart. Such coherently-prepared resonant gas exhibits high variation of the refractive index and large dispersion near the two-photon resonance. The light propagation through the media is then affected by considerably reduced group velocity, thus making the media of interest in communication systems for pulse delaying and compression and in optical memories.

 
Optical table

Experimentally, we investigate a laser-atom interaction by observing narrow CPT resonances and non-linear magneto-optical effects when multi-frequency laser light interacts with atomic vapour. The observation is made by detecting transmitted laser light or laser fluorescence, while lasers have different intensities, polarization and propagation directions. For current research, we use extended cavity diode lasers (ECDL) with different Rb vapor cells, with and without buffer gas, to measure the effects of the angular momentum of atomic levels coupled by laser, polarization and propagation directions of the lasers, cell size and gas density on the width of Raman or two-photon resonances.

EIT and EIA

CPT- Hanle effect is the effect of Zeeman splitting on CPT resonances in a near degenerate two-level atomic system. Depending on the angular momentum of coupled atomic levels, dark or brite states were obtained while measuring transmission of laser light. Dark state (bright state) appears as increased (decreased) transmission of electromagnetic radiation and decreased (increased) atomic fluorescence.

Hanle effect

Theoretically, we analyze CPT effects by solving optical Bloch equations for density matrix elements of atomic system and calculating the transsmisions of light.

We study photon interference as a process in which individual events (impacts) accumulate in time, and are recorded in recent experiments with one per one photon, in grating interferometers as well as in a Mach-Zehnder interferometer. These experiments are required from theorists in order to complete the standard theoretical description of interference.

The latter describes the interference patterns produced in the limit of very large (theoretically infinite) number of events. The theoretical description of intermediate patterns, corresponding to smaller number of events, is missing. The objective of our research is to find this missing theoretical description. We assume that particle and wave properties of photons are compatible and adapt the approach which we developed to describe the interference phenomena in the grating interferometers with massive particles - neutrons, atoms, molecules. This approach is based on the use of the time-dependent wave function of a particle's transverse motion and the probability amplitude of transverse momentum, assuming the classical motion in the longitudinal direction. Both near and far field behind a grating are considered.

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