From the source a 'spin up' photon is sent.
There are two observers, one is straight at the direction in which the photon was propogated and one is 45 degree to that direction. Both of them measure the state.
Now two questions.
1) What is the probability for each of the observer in seeing the state as 'spin up' ?
2) Is the first observer getting affected because a second observer is there ?

1)photons don't fly with spin up or somthing, they flying with random spin, so your questions unrelevant or unknow.
2)yes, but if spin is random, then no diferents you will not see, becouse first and second observer results will match 50% and missmatch 50%.

If you rotate at say 60 degrees, then match will be (cos(pi/3))^2=25, and missmatch probability will be (sin(pi/3))^2=75. And this violates bells inequalities (what is good for entanglement) and proves entanglement.

How is the observer who is located at a point 45 degrees to the direction in which the photon is travelling going to see it in the first place?

Hi!

Of course, a photon with known spin can be emitted. Radiotransmitters do it all the time.

Is the question about entanglement? This would need several photons. Can you describe it better then?

TheAmateur, could you clarify what you mean by "at the direction..."? Could it mean "with the same polarization"? If the second observer is off-path, he won't see the photon, as wrote Edward3.

If you emit a single photon and two observers are able of catching it, oh yes, the second observer will affect the first one: each time the second observer gets the photon, it's lost for the first one.

If you really mean entanglement: the presence of the second observer doesn't change what the first sees. Which implies they can't transmit information. Useful to save some theories, as the correlation between both observations is faster than their distance allows.

I was think that photon spin is it polarization. Anyway how spin is related with entanglement? I know that only polarization is related with entanglement.

Thanks all.
I was actually referring to bells experiment, where the source is at specific angle to the direction of travel of photons.

Enathalpy, I have a few more questions.
i) why entanglement needs more photons ?
ii) From your answer, I see that entangled photons could be seen by more than one observer at the same time, whereas a polarised photons could be seen by one person at a time. How?
iii) You told that the presence of one observer will not affect the other one. But in double slit experiment, when you add an observer to measure the photons from one of the slits, the entire system gets affected. How?

Maybe the point that may help you understanding is that "observing" a photon means destroying it. Or absorbing, or converting, call it as you want. But it's gone, finish, end.

So you need at least a photon for each observer to make them happy.

And adding an observer in a path means that the photon can't go through this path any more, so interferences between this path and the other vanishes.

As long as you think with photons, you lucky guy, just imagine them as lightwaves that appear and disappear by quanta multiples, and you get the right answer.

When the experiment means converting particles, QM becomes much more difficult to imagine than with photons. But it works, so you've no choice.

You may read Wiki as an introduction. And don't begin with entanglement, you must first understand more basic ideas like interferences.

DavidD, about spin and polarization: I misused "spin" for "polarization" as is commonly done. The spin of a photon is 1.

In double slit experiment photons aren't entangled or electrons. There is only one photon or electron.
For entanglement need (at least?) two photons. I know that very good experiments to obtain entanglement was maded with two photons.
I)for entanglement need two photon. One photon don't enough, becouse need to establish correlation.
II)for single photon there is experiement http://en.wikipedia.org/wiki/Mach-Zehnder_interferometer, which showing faster than light properties of single photon intereference in superposition. For entangled photons two observers is nessary to establish correlation and to know it's not a hidden variables.