# Nature of photon spin

1. Jul 18, 2011

### San K

by undefined do we mean 1 or 2 below?

1. we don't know the polarization (but it must be having some value of the spin) or

2. the photon does not have a definite spin yet

mixed polarization means: a mixture of defined spins? (up or down, right or left etc)

once a polarizer has defined a spin of a photon, how can one later undefine it?

for example can we undefine via having it strike a BBO crystal and allow two entangled photons to emerge with undefined spin?

thus a polarizer does two things?

1. forces undefined photons to define their state
2. a) filters (or splits) the recently defined photons (defined by the polarizer a micro micro micro micro micro micro micro micro micro second back)
2. b) filters (or splits) the "pre-defined" photons

so we have two kinds of photons. these photons are very different in terms of their state.

1. photons with defined spins. the spin will remain like that forever till it encounters things like BBO crystal

2. photons with undefined spins. the spin will remain undefined forever till it encounters things like a polarizer/QWPs etc

2. Jul 18, 2011

### xts

You have wrong view of photon spin vs. polarisation. Photon spin is always parallel to its trajectory. Pure states of spin +1 or -1 make a beam of circularily polarised light. Linearily polarised light is a mixed state of those. It is much easier to discuss all such issues using the base of linearly polarized photons, but then we shouldnt speak about spin.

Undefined polarisation :== we don't know the polarisation or we know it in some other base, than we currently use - e.g. we know photons passed already through clockwise circularil polarisator, but now we use linear polarisator to polarise them linearily in up-down plane.

Mixed polarisation :== 'that is ambigous - it may mean 'in a mixed state regarding currently selected base' or 'mixture of photons of which every one has different polarisation'

 once a polarizer has defined a spin of a photon, how can one later undefine it? ''
You may change it to another base. E.g. you may send linearily polarized beam through quartervawe plate to get circularily polarized beam.

Polarizers just filter the beam leaving only photons matching their polarisation axis.

We have only one kind of photons... We just know that some of them have some polarisation, but about other we don't have such knowledge.

Last edited: Jul 18, 2011
3. Jul 18, 2011

### San K

thanks again for answering the queries, you did an excellent job.

we don't have knowledge because:

1. we have not measured it yet
or
2. the polarization is undefined/indeterminate, and we will only know when we measure it because as soon as we try to measure it will become determinate

4. Jul 18, 2011

### San K

so we know its clockwise or anti, however we don't know if its right or left polarized? till we use the linear polarizer?