Transcript:

So we started looking at the double slit. So, what does it mean to observe the experiment? It means that you keep track of when each photon goes through which slit. Does it go through slit number A? Does it go through slit number B? This is what it means by ""to watch the experiment.""

Okay, but now let's make it a little bit more complicated. Now the slit is a quantum slit. It could be one slit, or it could be two slits, depending on the quantum state of the slit. And the photon is a quantum particle. And now, you run the experiment.

Okay, so here's the emission. The slit is here. So the quantum particle passes through the slit, and the screen is on this side. Okay, and that's what you do. But since it's a quantum slit, after the quantum particle has passed the slit—we have not observed it while it was passing—after it has passed the slit, we determine the quantum state of that slit. Then we know whether it's one slit or two slits after the fact. While it is passing, we have not observed. After the fact.

Now, interestingly, depending on what the measurement is of the quantum slit after it has passed it, on the screen, it's either an interference pattern if it is two slits, or there is no interference pattern if it is one slit.