What happens when one or both slits are closed after each photo has passed the slit but before it’s detected? In the double slit experiment in which single photons are fired at a pair of slits one at a time forming an interference pattern at the detector, what happens when one or both slits are closed after each photo is predicted to have passed the slits but before it’s detected?
 A: You could periodically cover one slit and from the timing information deduce whether a photon passed the system of two or one slits. Then you will find that the two types of photons build two different patterns, a one slit diffraction and a two slit combined diffraction and interference pattern. You will then trivially know that the first type went through the uncovered slit, but have no which way information about the second kind.
A: It makes sense to break down the experiment into simpler phenomena.
As I am sure you have been taught, these characteristic stripe-shaped intensity distributions of light are also formed behind a single slit. And they also form behind a single edge. You can convince yourself of this with another experiment at your institute.
The individually emitted photons also appear on the observation screen (the CCD chip) as individual points. Only after a longer observation phase will the statistical representation of the impinging points produce a stripe-shaped intensity distribution. The individual photons form dots on the screen, not waves and thus no interference.
It follows directly from the above that covering the edges after the passage of a photon has no influence at all on the impact of the photon. Rather, it makes sense to include the interaction between the edge(s) in the consideration.
A: The experiment can be done if the distance is far enough but one photon will not give you any kind of pattern. What you will find is the photons can be diverted left or right at the slit before hitting the screen. Closing the slit after has no effect on photons that have already passed.
