= Quantum optics
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<Quantum optics> studies <light> using <quantum states> of the <electromagnetic field> and their interaction with matter. <Photon statistics> asks how detections fluctuate; the <quantum theory of optical coherence> relates those detections to field correlations. A <coherent state>, <thermal light>, and a <Fock state> can have the same mean <photon> number yet different pair counts. This distinction makes <photon counting> useful for testing sources intended to emit one <photon> at a time.
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