# Diffraction of individual photons

1. Mar 10, 2013

### gespex

Hi all,

If we have any setup that causes diffraction with light, such as a single slit, and count the number of photons detected at a certain point behind this slit, is it true that we can determine this count as follows:
1. Consider each point in the slits a new wave source, and calculate for the detected point the amplitude, A, of the sum of all these new waves.
2. The number of detected photons will be proportional to the square of this amplitude, so proportional to A^2.

2. Mar 10, 2013

### chill_factor

I believe photon number N is not an invariant quantity except for Fock states.

3. Mar 11, 2013

### Zarqon

If you are talking about the average or expected photon count (not necessarily exact count) then yes, it should be right what you wrote.

4. Mar 11, 2013

### gespex

Thanks for the replies, but which of the two is it ;-)? Zarqon: yes, I mean the average/expected photon count. So who of you two is right? Or both: I'm not sure what Fock states are, so maybe Zarqon considered that to be implicit?

5. Mar 12, 2013

### Zarqon

Fock state = exact photon number is known (but the phase of the light is not)

chill_factor simply pointed out that you will not be able to get the exact photon number unless you prepare your light in a very particular state (and Fock states are generally difficult to create). And then I continued on that saying, what you wrote is ok if you don't need the exact photon count, but are happy with the average. So there was no disagreement. Hope that clears it up.

6. Mar 12, 2013

### gespex

Ok, got it, thanks!