0
$\begingroup$

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:

  • 3 real-valued parameters to describe its x,y,z direction in space.
  • 1 real-valued parameter to describe its frequency, or equivalently, its energy.
  • 1 real-valued parameter to describe its phase offset.
  • 1 integer-valued parameter to describe its spin angular momentum, which is either +1 or -1 times ħ.

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: Photon electric and magnetic fields

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:

  • The integer-valued parameter to describe its orbital angular momentum.
  • The real-valued orientation (rotation) of the photon's fields relative to the reference point.

...

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!

$\endgroup$
1
1
$\begingroup$

To completely describe the state of a single photon one needs the following information:

  1. The polarization state (one value)
  2. The energy (one value)
  3. The momentum (the energy-momentum four-vector is equivalent to the position four-vector), which requires three values.
  4. The orbital momentum state (one value)

Which means there are six degrees of freedom for a single photon.

$\endgroup$
2
  • $\begingroup$ Wouldn't you also include the phase of its wavefunction (relative to some reference) as a 7th degree of freedom? Every description of the wave vector for a photon I've seen includes a phase parameter. $\endgroup$
    – antiduh
    Aug 11 '20 at 13:55
  • $\begingroup$ I don't think so. That's the same as including three position values (or, equivalently, three momentum values) and the energy (wavelength). $\endgroup$ Aug 11 '20 at 14:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.