Physics Asked by antiduh on March 7, 2021
I’m trying to understand what parameters uniquely define the state of a photon, and I can’t seem to find a straightforward reference anywhere. Wikipedia is either unclear here, or over my head.
If you were to restrict yourself to parameters that are integer-valued or real-valued (if this is possible), how many parameters would you need to describe a photon from a fixed reference point?
The best I can come up with is at least 6, but I’m unsure about some of this. Here’s what I have:
There are two more parameters that I’m unsure about.
First, Wikipedia says "each photon carries two distinct and independent forms of angular momentum" and lists spin angular momentum (SAM) and orbital angular momentum (OAM). SAM seems to pretty clearly be a property of a single photon, however, the language used in the Photon article and related articles when discussing OAM makes it sound like OAM might be a property of a set of photons, and not just of a single photon. Is it a property of a single photon?
Second, photons are often depicted as a magnetic field and an electric field oriented 90 degrees from each other, perpendicular to the direction of travel of a photon, such as this image:
Can that field have any orientation relative to some fixed reference? Can I have two photons where one’s fields are perfectly aligned with my frame of reference, but the other is rotated?
Depending on the answers to the above, the tally comes to 8 parameters:
…
My apologies if the question itself contains mistruths or misunderstandings. My background is in computer science, and I work for an RF engineering company which is where my curiosity stems from.
…
Edit: After reading through the link provided by Emilio Pisanty, it appears that there are two forms of orbital angular momentum – external and internal, where internal refers to the orbital angular momentum of a single photon. Thanks Emilio!
To completely describe the state of a single photon one needs the following information:
Which means there are six degrees of freedom for a single photon.
Correct answer by Deschele Schilder on March 7, 2021
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