Quantum interference effects in closed-loop atomic systems

When laser radiation excites several transitions between quantum states in an atom, these transitions are able to interfere. Such a quantum interference is the basis for numerous interesting phenomena and innovations in laser, atomic and molecular physics. For example, when the interference is destructive, an atom does not absorb light at all in spite of the fact that it is resonantly excited. Correspondingly, the laser light propagates loss-free through a medium comprised of such "dark" atoms. The effect is called Electromagnetically Induced Transparency (EIT). At specific values of laser frequencies, the interference is completely destructive, and hence EIT takes place. It is this sharp resonant response of the medium to radiation that is used in many applications of EIT in precise magnetometry, optical communications, non-linear optics, quantum information, etc. Here the interaction of multi-frequency electromagnetic radiation with a dense medium of atoms where the radiation-induced transitions form a closed loop was investigated. In closed-loop systems, manifestation of EIT and other interference effects depends resonantly not only on the relative frequency of the applied fields, but also on their relative phase and relative intensity. Therefore, new possibilities for controlled manipulation of both the atoms and the fields, and new potential applications appear as compared to ordinary, "open" systems. In the frame of the present project, we have predicted, theoretically described and for the first time experimentally demonstrated the effect of phase-dependent EIT. A medium absorption that can be controlled by laser phases has never been observed before in any optical material, and it may have important applications in precise measurement of very weak interactions (magnetometry, gravitational wave detectors, etc.). Additionally, it represents a basis for new types of opto-electronic devices such as optical switches and phase-amplitude modulators. Another promising application of EIT - non-linear-optical generation of radiation - uses the reduction of absorption of both pump and generated fields in order to enhance the efficiency and to reduce the intensity threshold for the process. We could experimentally observe a very efficient generation of "Raman sidebands" of pump fields, that is a series of light field components with frequencies separated by a frequency of Raman transition in atoms. The observed power threshold of 10 microwatts is one of the lowest ever observed. This achievement paves the way for non-linear optics with few photons. Theoretically, we have predicted that EIT in closed-loop systems can be used to generate continuous-wave terahertz radiation. One more experimental work of this project was concerned with the noise correlation of two or more frequency components of laser radiation. It was experimentally demonstrated for the first time that EIT allows such a correlation, with EIT in closed-loop systems performing in a much wider spectrum of noise frequencies. This work may have considerable impact in laser physics, and is a necessary step towards fields correlation on a quantum level, which is important for fundamental studies in quantum physics and for quantum information processing.