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Electromagnetically induced transparency
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<p>Electromagnetically induced transparency (EIT) is a <a href="/facts/Coherence_(physics)/NF1Rgdoa">coherent</a> <a href="/facts/Optics/p0vq4S1T">optical</a> <a href="/facts/Nonlinear_optics/maerngXd">nonlinearity</a> which renders a medium <a href="/facts/Transparency_(optics)/PRryNNW6">transparent</a> within a narrow <a href="/facts/Spectrum/wPUjkpqI">spectral</a> range around an <a href="/facts/Absorption_line/HuI4Bj5f">absorption line</a>. Extreme <a href="/facts/Dispersion_(optics)/cUdHnYXz">dispersion</a> is also created within this transparency "window" which leads to "<a href="/facts/Slow_light/ImIjVmQj">slow light</a>", described below. It is in essence a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium. </p><p>Observation of EIT involves two optical fields (highly coherent light sources, such as <a href="/facts/Laser/jk6PTvvT">lasers</a>) which are tuned to interact with three <a href="/facts/Quantum_state/7jbxd7Dx">quantum states</a> of a material. The "probe" field is tuned near resonance between two of the states and measures the <a href="/facts/Absorption_spectrum/GH7RC14D">absorption spectrum</a> of the transition. A much stronger "coupling" field is tuned near resonance at a different transition. If the states are selected properly, the presence of the coupling field will create a spectral "window" of transparency which will be detected by the probe. The coupling laser is sometimes referred to as the "control" or "pump", the latter in analogy to incoherent optical nonlinearities such as spectral hole burning or saturation. </p><p>EIT is based on the destructive interference of the transition <a href="/facts/Probability_amplitude/ryPgJvs8">probability amplitude</a> between atomic states. Closely related to EIT are <a href="/facts/Coherent_population_trapping/QXnCwI5b">coherent population trapping</a> (CPT) phenomena. </p><p>The quantum interference in EIT can be exploited to <a href="/facts/Laser_cooling/hU2c6tdk">laser cool</a> atomic particles, even down to the quantum mechanical ground state of motion. This was used in 2015 to directly image individual atoms trapped in an <a href="/facts/Optical_lattice/Inu8klUj">optical lattice</a>. </p>