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Author Topic: Do Individual photons posess bandwidth?  (Read 3732 times)

Offline Soul Surfer

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Do Individual photons posess bandwidth?
« on: 29/11/2005 11:39:51 »
This is a question that thas stumped most of the serious scientists that I have asked.  Their normal response is to say that is a really good question and that they would like to know too.

I would visualise a narrow band photon as one that was a long wave train and a broad band one just a few waves long.

I know that the emission and absorption processes for photons can be have different bandwidths because their spectral lines can have inherently different widths. The spectral lines from very short lived processes are broad because the short time introduces uncertanty in frequency but emissions from long lived metastable states are very narrow and used as frequency references.

Normal spectroscopes require many photons to determine the statistics of their precise frequency but, is it in theory possible to measure this property in a single photon?  If so bandwidth is another property of a photon that must be included in its description and effectively increases its information content.

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Offline daveshorts

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Re: Do Individual photons posess bandwidth?
« Reply #1 on: 29/11/2005 16:09:41 »
The momentum of a photon is related to it's frequency, so an uncertainty in it's momentum will lead to a bandwidth. So from heisenberg's uncertainty principle as you say the better you know the position of the photon the bigger the bandwidth... I think that was just telling you back what you told us... oh well
 

another_someone

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Re: Do Individual photons posess bandwidth?
« Reply #2 on: 30/11/2005 09:23:56 »
quote:
Originally posted by daveshorts

The momentum of a photon is related to it's frequency, so an uncertainty in it's momentum will lead to a bandwidth. So from heisenberg's uncertainty principle as you say the better you know the position of the photon the bigger the bandwidth... I think that was just telling you back what you told us... oh well



If this is true, then can we really say that bandwidth is new information?  If bandwidth is a measure of uncertainty in position, and uncertainty in position is an already known quantity, then is not bandwidth merely a manifestation of something we already know, and not something that is separate from the information we already know?
 

Offline Soul Surfer

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Re: Do Individual photons posess bandwidth?
« Reply #3 on: 02/12/2005 11:35:51 »

If bandwidth were an individually measurable property of a photon you could (in theory) distinguish between two photons of nominally the same frequency but different bandwidths coming from different sources in the same direction at similar times.

This is in contrast to an electron where all electrons are indistinuishable from each other except for the binary property of polarization.  Photons are also distinguishable by polarisation.

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Offline aktawa

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Re: Do Individual photons posess bandwidth?
« Reply #4 on: 12/12/2005 04:37:27 »
A photon is actually only a excitation of a free space mode, i.e. like an atom can have an excited state, the electromagentic field is quantizised to. Each mode has a frequency.

If you are not in a cavity, there are infinite modes, which are infinite close together. If you are trying to generate one photon, you will excite a bundle of photons with a properbility. So you will have a bandwith.

In a cavity (depending on the border conditions) you will get a frequency spacing between the modes. Therfore you can only excite one mode. (in reality, you will always measure a bandwith at least to the fact, that you cannot wait for eternity for waiting to measure the photon)
 

Offline Soul Surfer

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Re: Do Individual photons posess bandwidth?
« Reply #5 on: 12/12/2005 09:28:25 »
What you say is not very clear.  

I am well aware of the "cavity" analogy related to photon emission and detection which comes from the basic mathematics of quantum mechanics.  

I am also well aware of the relationship between time and bandwidth in which a narrow band process requires longer to emit and detect.  I stated this in my original presentation of this question.  

I also have at no time suggested that I was talking about zero bandwidth (and an inifinitely long wait)as you mention in your last paragraph.

What I think that you may be trying to say is that bandwidth is a property of the source and the detector independantly of the photon which does not have a defined bandwith while it is moving.  This I agree is probably the answer to the question but I have never seen it explicitly stated in a textbook.  The relationship between bandwidth and information is often glossed over (I have made a good chunk of my career in knowing about this and exploiting it to the full!).

This explanation would fit well with the peculiarities of quantum mechanics and produce results in agreement with the observed facts but it plays havoc with our mental images of photons as little electromagnetic wavetrains because ANY photon must simultaneously have states with a great many waves and a very few all superimposed upon each other.


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Re: Do Individual photons posess bandwidth?
« Reply #5 on: 12/12/2005 09:28:25 »

 

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