0

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?

diff
  • 681
  • 3
    related: http://physics.stackexchange.com/questions/89018/are-coherent-states-of-light-classical-or-quantum?rq=1 – Rococo Jun 23 '16 at 22:26
  • That link by Rococo, at least one of the answers, indicates that there is a difference between classical and quantum coherent states. Tv and radio signals are classically coherent (to an extent, they have a coherence BW and time) but not quantum coherent states. They are produced by oscillators with small phase noise, but they do have phase noise and noise BW more than any microwave spectral lines, and are not produced by quantum emitters. Masers are a different thing, like lasers, quantum coherent (again to some extent). The two coherences are qualitatively and quantitatively different. – Bob Bee Jun 23 '16 at 23:26
  • 1
    @BobBee In your view, what experiment would distinguish a radio wave generated by a standard (classical) oscillator from a maser at the same wavelength? Is it just the bandwidth? – Rococo Jun 23 '16 at 23:40
  • Not classical double slit or for rf interference. There was plenty rf 'interference' fringes before QM. It a matter of how generated and how much coherence. The how generated you'd say well it's classical/macroscopic. But there is nothing quantum, it is a coherent classical oscillator with little phase noise and high Q. No comparisons to the quantum effects in eg an rf squid or maser. If you want a strict answer ask separately a strict and well defined different question – Bob Bee Jun 25 '16 at 20:30

1 Answers1

1

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
  • 7,671
  • 1
  • 23
  • 54
  • I understand that a coherent quantum state can exhibit quantum interference. Is the electron before passing through the two slit in double slit experiment a coherent quantum state? – diff Jun 23 '16 at 23:20
  • An fm signal is not quantum coherent. See my comment above – Bob Bee Jun 23 '16 at 23:27
  • @diff: In general, no. A 'coherent quantum state' is the name for a particular type of state, but it is not the only type of state to have 'quantum coherence' in the sense of, for example, showing interference fringes in a double slit experiment. The terminology is confusing, I know! – Rococo Jun 23 '16 at 23:52