## Amount of energy required to change the spin of a photon

Not sure if the question makes sense, nevertheless it can help clarify some concepts, I guess.

What is the amount of energy required to change the spin (intrinsic angular momentum) of a photon?

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 Zero. You can simply pass the photon through a half wave plate. Afterwards the photon still has the same energy; you don't need to do any work. However, there is a transfer of angular momentum between the wave plate and the photon.

 Quote by The_Duck Zero. You can simply pass the photon through a half wave plate. Afterwards the photon still has the same energy; you don't need to do any work. However, there is a transfer of angular momentum between the wave plate and the photon.
Thus ....is the transfer of angular momentum, in this case, friction-less?

## Amount of energy required to change the spin of a photon

well if you are talking about making a spin 1 into spin 1/2 ,then it is not possible.

 Quote by andrien well if you are talking about making a spin 1 into spin 1/2 ,then it is not possible.
Hi Andrien, I am talking about changing the spin (along a particular axis) from, say, spin up to spin down.

 Blog Entries: 1 Recognitions: Science Advisor There are only two things you can do to a photon: a) You can emit it. b) You can absorb it. You cannot change its spin. The comments above apply to a light ray, in the classical sense, but not to an individual photon. You cannot pass a photon through a half wave plate, or focus it with a lens, or bounce it off a mirror. These are collective interactions with the atoms of a solid object. A photon is simply absorbed by the first thing it encounters.

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 Quote by Bill_K You cannot change its spin. The comments above apply to a light ray, in the classical sense, but not to an individual photon. You cannot pass a photon through a half wave plate, or focus it with a lens, or bounce it off a mirror. These are collective interactions with the atoms of a solid object. A photon is simply absorbed by the first thing it encounters.
Hmm, I suppose all the people working on polarization entangled photons and polarization properties of single photon sources should retract all their papers then because they pass single photon states through quarter and half wave plates all the time. They focus them and use mirrors on them all the time. Of course one can do all of that with single photon states. See e.g. Phys. Rev. Lett. 86, 1502–1505 (2001) where all of this is done with single photons.

 Blog Entries: 1 Recognitions: Science Advisor Interesting. Please draw me the Feynman diagram of a photon interacting with a quarter wave plate.


Surely this is just a disagreement over the word "photon." I'm sure everyone agrees on the physics if not the terminology. If by photon we mean "a single wavy line in a Feynman diagram" then we can only emit it or absorb it and the spin of a single photon can't be changed. If by photon we mean something like "a state that, when it interacts with a photon detector, causes one count" [what's a better way of stating this?] so that we include collective excitations within transparent materials, then we can speak of sending a photon through a wave plate or focusing it with a mirror.

In any case, to answer the original question, everyone should agree that we can go from a state with a positive helicity photon to a state with a negative helicity photon at no energy cost.

 Quote by San K Thus ....is the transfer of angular momentum, in this case, friction-less?
Well, I imagine we can't get perfect efficiency. Some of our photons will be absorbed or reflected by the wave plate instead of being transmitted. But not very many (for a well-constructed wave plate) and I think you could get arbitrarily close to perfect efficiency by improving the quality of your wave plate.

 In any case, to answer the original question, everyone should agree that we can go from a state with a positive helicity photon to a state with a negative helicity photon at no energy cost.
Do you have a reference for that,because it is really silly.

 Quote by andrien Do you have a reference for that,because it is really silly.
This is what a half-wave plate does, as I mentioned in my first post above. Wikipedia explains
 Quote by Wikipedia the effect of the half-wave plate is to mirror the wave's polarization vector through [a certain plane]. For linearly polarized light, this is equivalent to saying that the effect of the half-wave plate is to rotate the polarization vector through an angle 2θ; however, for elliptically polarized light the half-wave plate also has the effect of inverting the light's handedness.
As you mentioned above, the photon does remain a spin 1 particle; it's the direction of the spin that changes. A half-wave plate changes left circularly polarized photons to right circularly polarized photons and vice versa. Circularly polarized photons are in states of definite spin/helicity, and inverting the handedness of the polarization switches the spin/helicity.

 Well I was really thinking in terms of some quantum mechanical situation because spin is nevertheless a quantum phenomenon,however it is possible to prove that light does have a spin 1 character but that really goes with circular polarization and not plane polarized .So I was thinking about some quantum mechanical way of doing it ,if it is there.

 Quote by Bill_K There are only two things you can do to a photon: a) You can emit it. b) You can absorb it. A photon is simply absorbed by the first thing it encounters.
thanks Bill_K. Is this process (emittance, absorption, re-emittance, re-absorption) friction-less?

 Quote by The_Duck In any case, to answer the original question, everyone should agree that we can go from a state with a positive helicity photon to a state with a negative helicity photon at no energy cost.
Thanks for the information Duck.

In classical physics, we are used to the idea that any kind of change must involve energy (transfer/use).

So the above, like many other quantum mechanical phenomena, is difficult to comprehend in the Newtonian-Einsteinian mindset.

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