Photon Emission from Antennas: Explained

[CosmicVoyager]

Greetings,

As much as I know about science, sometimes something very fundamental dawns on me that I do not understand.

I thought all photons were emitted by electrons. Having recently been pursuing interest in radio communication, I realized I do not understand how an antenna produces a photon.

If I have a vertical antenna rod a few feet long to broadcast and receive microwaves, how exactly does it happen? Considering a single photon, at what point on the antenna is it emitted and why? Is it from a random point?

If you say it is emitted from the entire antenna at once, how can that be since the electric current has to flow from through the antenna? At what point in the leading edge of the current traveling along the antenna is the photon emitted? It does not travel at infinite speed, right? And is the photon centered at the center of the antenna?

I read the Wikipedia articles on photons, electromagnetic radiation, and antennas, and none of them explain this.

I would like to know in detail what occurs from there be no electricity in the antenna, through turning it on, through a photon being emitted.

Thanks

 

[SpectraCat]

Greetings,

As much as I know about science, sometimes something very fundamental dawns on me that I do not understand.

I thought all photons were emitted by electrons. Having recently been pursuing interest in radio communication, I realized I do not understand how an antenna produces a photon.

If I have a vertical antenna rod a few feet long to broadcast and receive microwaves, how exactly does it happen? Considering a single photon, at what point on the antenna is it emitted and why? Is it from a random point?

If you say it is emitted from the entire antenna at once, how can that be since the electric current has to flow from through the antenna? At what point in the leading edge of the current traveling along the antenna is the photon emitted? It does not travel at infinite speed, right? And is the photon centered at the center of the antenna?

I would like to know in detail what occurs from there be no electricity in the antenna, through turning it on, through a photon being emitted.

Thanks

This might help get you started. http://dept.kent.edu/projects/ksuviz/leeviz/dipole/dipole.html

 

[CosmicVoyager]

Wow. That could not have been more disappointing. LOL Thanks, but it said absolutely nothing about what I want to know :-(

Glad to see this was moved to quantum physics. I had a feeling it had something to do with that. I am familiar with quantum physics: particles in multiple states until measured; quantum entanglement; etc.

 

[SpectraCat]

Wow. That could not have been more disappointing. LOL Thanks, but it said absolutely nothing about what I want to know :-(

That's funny, because it directly addresses a couple of the points you asked about.

Glad to see this was moved to quantum physics. I had a feeling it had something to do with that. I am familiar with quantum physics: particles in multiple states until measured; quantum entanglement; etc.

The questions you asked about single photons being emitted from "points" on the antenna aren't really germane for how real antennas work, which is what I thought you were interested in. To properly describe emission of single photons from oscillating charges, you need to use quantum field theory. However, I don't believe that the qualitative aspects of the description are very different from those of a classical dipole. Sorry you didn't find my link more helpful.

 

[Antiphon]

An antenna is capable of emitting a single photon. All you have to do is turn the intensity of the transmitter way down.

Antennas work by making the conduction electron "gas" rattle around the antenna structure. The electrons aren't free to leave the structure but they are free to move on it's surface- much more freely than an electron in a valence band around an atom. As a result of this freedom, the antenna can shoot photons out in ways an atom can't, such as at very low energies or even over a continuous spectrum, rather than the discrete emission lines of atoms.

When an antenna emits a single photon, the whole antenna participates, not just some small portion of it.

 

[CosmicVoyager]

An antenna is capable of emitting a single photon. All you have to do is turn the intensity of the transmitter way down.

Antennas work by making the conduction electron "gas" rattle around the antenna structure. The electrons aren't free to leave the structure but they are free to move on it's surface- much more freely than an electron in a valence band around an atom. As a result of this freedom, the antenna can shoot photons out in ways an atom can't, such as at very low energies or even over a continuous spectrum, rather than the discrete emission lines of atoms.

When an antenna emits a single photon, the whole antenna participates, not just some small portion of it.

Thanks. That is very helpful.

1 - Is the photon centered at the center of the antenna. That is, is the axis around which the electric and magnetic fields of the photon oscillate at the center of the antenna rod? Or is something quantum mechanical going on like the photon not having a location but being in a superposition of having emitted at every point along the antenna?

2 - Also, can you explain what is going on with the electric current? When you turn on the power, the electrons flow through the antenna near the speed for light? Does the current have to alternate and be reversed at least once to emit a photon?

3 - The frequency of the photon is determined by the rate at which the current alternates, right? So can will any length antenna emit any frequency? I am wondering why different length antennas are used for different frequencies, exactly.

Do you know of a source that explains this conceptually rather than for practical use?

 

[vanhees71]

Unfortunately this statement is not true also often made in attempts of popular-science writers to explain what a photon is. You cannot prepare single-photon states by simply dimming down a Laser or a radio transmitter in intensity. By this you'll still always get a coherent state. At very low intensity (which can be tuned down such that the average photon number becomes even lower than 1 photon!) this coherent state is approximately a superposition of the vacuum (no photons) and a single-photon state (1 photon). Thus, in most cases, you'll simply find "no photon" and sometimes "one photon". To find "two photons" becomes very unlikely then, but it's not impossible. The photon-number statistics is approximately Poissonian, and you can easily calculate the probability to find N photons when the average number of photons is given.

Nowadays it's not so difficult anymore to create true single-photon Fock states since the quantum opticians have found crystals which can be used to create entangled two-photon states by a non-linear optics effect known as parametric downconversion. Thus, you can use one of these photons as a trigger photon to make sure you have the other photon in the pair. Since the two photons are entangled (with respect to their polarization state), by measuring the polarization of the trigger photon you even get polarized single-photon beams in this way. This has lead to a large number of high-precision experiments which could demonstrate the oddest quantum behavior predicted by quantum field theory. So they could build teleportation devices, demonstrate the violation of Bell's inequality or construct a quantum eraser.