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I have a very fundamental question. We explain polarisation of light assuming wave nature of light. Is it still valid if we assume light as photons? Or in other words, polarisation is a wave concept or applicable to individual photons as well?

May be this is my wrong concept! I feel some confusion here.

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Photons are particles of spin $1$. In quantum mechanics, measuring the spin of such a particle can yield three results: $+\hbar$, $0$ or $-\hbar$. However the photon is a particular case because its mass is equal to $0$ and quantum field theory excludes this value for massless particles, so it can only be $+\hbar$ or $-\hbar$. Moreover this spin can only be measured in the direction of propagation, so its get the name helicity. The two helicities of the photon correspond to the circular polarisations of light.

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  • $\begingroup$ Ok. So, when an unpolarised light pass through a polariser, according to wave concept, the electric field oscillations confine into a single plane. How can we explain this with photon concept? Could you please explain a little bit more? Thanks. $\endgroup$
    – albedo
    Commented Jan 9, 2014 at 8:57
  • $\begingroup$ Also, what is the helicity of an unpolarised light? $\endgroup$
    – albedo
    Commented Jan 9, 2014 at 9:03
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    $\begingroup$ @albedo. Unpolarized light does not exist at the photon's level. It is a macroscopic concept. Unpolarized light is made of a uniform distribution of all polarized states. Yet, if you want to understand it in terms of photons, you should think of a large number of photons with left-handed and right-handed helicities in the same amount and uniformly distributed phases. $\endgroup$
    – Tom-Tom
    Commented Jan 9, 2014 at 9:09
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    $\begingroup$ @V.Rossetto Actually you can understand depolarisation at the one photon level: the photon is in a mixed state defined by a density matrix. I actually think the quantum description of depolarisation is a great deal easier than the classical: your comment on the ensemble description is also elegant and a great way to think of depolarised light. When you want to understand it in terms of classical waves, it gets quite thorny - it's the same problem with partially coherent light (which also has a cleaner quantum desription) and Born and Wolf give a whole chapter to the classical description. $\endgroup$
    – Selene Routley
    Commented Jan 9, 2014 at 12:27
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    $\begingroup$ @V.Rossetto BTW I nearly forgot +1! Also, the density matrix description for a photon in a mixed state is equivalent to the definition of its Stokes parameters, see my answer here physics.stackexchange.com/a/92176/26076 if you're interested. $\endgroup$
    – Selene Routley
    Commented Jan 9, 2014 at 12:30

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