Editing Coherence (physics) (section)

=== Macroscopic quantum coherence ===
[[Macroscopic scale]] quantum coherence leads to novel phenomena, the so-called [[macroscopic quantum phenomena]]. For instance, the [[laser]], [[superconductivity]] and [[superfluidity]] are examples of highly coherent quantum systems whose effects are evident at the macroscopic scale. The macroscopic quantum coherence (off-diagonal long-range order, ODLRO)<ref>{{cite journal
 |last1=Penrose |first1=O.
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 |year=1956
 |journal=Phys. Rev.
 |volume=104 |issue=3
 |pages=576–584
 |title=Bose-Einstein Condensation and Liquid Helium
 |doi=10.1103/physrev.104.576
|bibcode=1956PhRv..104..576P}}</ref><ref>{{cite journal
 |last1=Yang |first1=C.N.
 |year=1962
 |journal=Rev. Mod. Phys.
 |volume=34 |issue=4
 |pages=694–704
 |title=Concept of Off-Diagonal Long-Range Order and the Quantum Phases of Liquid He and of Superconductors
 |doi=10.1103/revmodphys.34.694
|bibcode=1962RvMP...34..694Y}}</ref> for superfluidity, and laser light, is related to first-order (1-body) coherence/ODLRO, while superconductivity is related to second-order coherence/ODLRO. (For fermions, such as electrons, only even orders of coherence/ODLRO are possible.) For bosons, a [[Bose–Einstein condensate]] is an example of a system exhibiting macroscopic quantum coherence through a multiple occupied single-particle state.

The classical electromagnetic field exhibits macroscopic quantum coherence. The most obvious example is the carrier signal for radio and TV. They satisfy [[Roy J. Glauber|Glauber]]'s quantum description of coherence.