Coherent states 6.1 | Quantum coherence, 6.1

Every-day electromagnetic radiation, such as radio and TV waves, is also an example of near coherent states (macroscopic quantum coherence). That should “give one pause” regarding the conventional demarcation between quantum and classical.

— Wikipedia on Coherent states

In laser, all photons are in-phase with each other.

In other electromagnetic waves, the photons are out of phase. However, they still have constant phase difference (aka coherence), unless the photons are emitted by thermal radiation (thermal light). 

— Me@2011.10.06

However, such a picture is incorrect.

— Me@2011.10.12

In classical optics light is thought of as electromagnetic waves radiating from a source. Often, coherent laser light is thought of as light that is emitted by many such sources that are in phase. Actually, the picture of one photon being in-phase with another is not valid in quantum theory.

— Wikipedia on Coherent states

2011.10.12 Wednesday (c) All rights reserved by ACHK