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In quantum coherence wikipedia article

"Regarding the occurrence of quantum coherence at a macroscopic level, it is interesting to note that the classical electromagnetic field exhibits macroscopic quantum coherence. The most obvious example is the carrier signal for radio and TV. They satisfy Glauber's quantum description of coherence."

As far as I understood, classical electromagnetic field is a thermal state, which is a mixed state. Why does it exhibit macroscopic quantum coherence?

Ka Wa Yip
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1 Answers1

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Two points:

  1. A radio or tv signal is certainly not a thermal state. A thermal EM state looks like blackbody radiation, and is indeed incoherent.

  2. An EM field that has a well-defined phase and frequency, like an FM radio wave or a monochromatic laser, is certainly coherent. Whether it should be said to have 'quantum coherence' is debatable. It could be described by a coherent quantum state (see the link above), but also typically by a coherent configuration of the classical EM field. As a result, people will occasionally get in various debates over whether it is exhibiting 'quantum' or 'classical' behavior that really depend more on precisely what your definition of 'quantum' is than anything about the system itself.

Rococo
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