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Q & A: photon identity

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Most recent answer: 09/18/2012
Is the photon particle I see here on Earth the same one that left the distant star billions of Light years away?
- Frank
Hurst , Tx
If somehow a star emitted a blip of (approximately) a single photon, then when it arrived here I think you could say it was the same photon that was emitted, just like for any other particle.

In practice, the light emitted from stars (or light bulbs, etc.)  is in a state with lots of photons jumbled together. Such states don't really have individual photons. In fact, in general they do not even have a well-defined number of photons. The photon number is one of those many quantum variables (like position or momentum) which has a range of values.  When you put a photon-counting detector in it, the number of counts you get is not completely predictable.

Mike W.

(published on 11/18/2010)

Follow-Up #1: When can you count photons?

"The photon number is one of those many quantum variables (like position or momentum) which has a range of values." ->This sounds strange. Under what conditions can you count photon (or cannot)? You can count a photon only when it is traveling alone? (oh, one photon came and here is another.) but if there are more than one propagating together, the photon number can take any continuous value like (oh, there are 7.8415 photons or π/e photons etc.)? Why does the photon number not come in integer multiple when traveling in bundle? (or it does? but you just cannot measure it?) For example, if there is a single photon emitter, can you put two of them so that they emit a single photon from each at once and you get two photons propagating together? And, can a photon "radiate" from an emitter that always produces "one photon" at a time? or the single photon always travel toward one direction and cannot spread out as it travels? A single quantum of light (a photon) does not split(propagates) into two photons (with lower energy/frequency) as it travels?
- Anonymous
You can in general count photons when you have a photon-counting measuring device, such as certain types of diodes or photomultiplier tubes. Just to clarify for other readers, what you're asking here is for what sort of states of the photon field can you predict the number of photons to be counted with high accuracy.

The key issue is not whether the expected number of photons is one or something larger. Of course if the expected number is zero, the prediction is easy.

For standard sources the average photon number can be anything, not just an integer, although the count from your measuring device will be an integer.

 For special single-photon emitters, which can be made using small electronic devices, the combined output of a collection of these still has a well-defined photon number.

At the opposite limit, devices which produce well-defined electromagnetic fields have large uncertainties in the photon number, approximately the square root of the expected number. The phase of the radiation field doesn't commute with the photon number, so there's an uncertainty relation requiring that they don't both have definite values.

There's no general principle requiring some particular spatial pattern for a single-photon emitter. The simplest sorts of single-photon emitters, say a single atom in an excited state, tend to have some multi-pole pattern, say dipole. There's major uncertainty in where the photon will be found.

A photon cannot split into two photons without interacting with something else. How would it conserve angular momentum? The simplest all-photon scattering processes require two in and two out. For a discussion, see

P.s. You've sent in many highly sophisticated questions, right at the limit of what we can handle. I'm curious as to what your story is. I halfway expect at some point to get a grade on this test.

Mike W.

(published on 09/18/2012)

Follow-up on this answer.