Interference pattern observed half-way into the formation

[avarmaavarma]

We know that the interference pattern forms even when photons (or electrons etc.) are sent one at a time.

The photons only land on the fringe parts of the screen and not the dark part - as if directed by some underlying information about where they are supposed to go. So far so good.

We also know that if you actually try and gather any information (i.e. observe in any way),
then the pattern disappears and we get individual spots on the screen.

So - is there a way to trick the pattern formation as follows:

Step 1: Let the pattern form - i.e. do not observe the photons (going out one at a time), so that the pattern starts forming.
Step 2: Mid-way into our experiment - make an observation - so that the interference is destroyed. What appears on the screen? The interference pattern was already forming - so do we see a 'half-formed' interference pattern?

Does the pattern disappear altogether? If so - how is that possible - since the pattern was already half formed?

Any thoughts?

 

[Zarqon]

I think you can just looked at it one particle at the time. That is, any early particle that has gone through the slit unobserved, will contribute to the interference pattern, and and later particle that is subject to observation, will contribute to a more uniform pattern. So if you look at the total pattern afterwards, you will see an interference pattern that has a lower contrast than you would see if only unobserved particles were used, meaning the difference between peak and valley of the pattern is smaller.