Medium Requirements
There are specific restrictions on the configuration of the three states. Two of the three possible transitions between the states must be "dipole allowed", i.e. the transitions can be induced by an oscillating electric field. The third transition must be "dipole forbidden." One of the three states is connected to the other two by the two optical fields. The three types of EIT schemes are differentiated by the energy differences between this state and the other two. The schemes are the ladder, vee, and lambda. Any real material system may contain many triplets of states which could theoretically support EIT, but there are several practical limitations on which levels can actually be used.
Also important are the dephasing rates of the individual states. In any real system at finite temperature there are processes which cause a scrambling of the phase of the quantum states. In the gas phase, this means usually collisions. In solids, dephasing is due to interaction of the electronic states with the host lattice. The dephasing of state is especially important, ideally should be a robust, metastable state.
Current EIT research uses atomic systems in dilute gasses, solid solutions, or more exotic states such as Bose-Einstein condensate. Work is also being done in semiconductor nanostructures such as quantum wells, quantum wires and quantum dots.
Read more about this topic: Electromagnetically Induced Transparency
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