It you place a detector on one of the two slits in the prior experiment, (so that you measure which slit each individual photon goes through) the interference pattern disappears.
If you leave the detector in place, but don't record the data that was measured, the interference pattern is back.
This is not remotely true. It looks like you read an explanation of the quantum eraser experiment that was either flawed or very badly written, and you're now giving a mangled account of it.
A lot of people pose it as a question of pure information: do you record the data or not?
But what does that mean? The “detector” isn’t physically linked to anything else? Or we fully physically record the data and we look at it in one case vs deliberately not looking in the other? Or what if we construct a scenario where it is “recorded” but encrypted with keys we don’t have?
People are very quick to ascribe highly unintuitive, nearly mystical capabilities with respect to “information” to the experiment but exactly where in the setup they define “information” to begin to exist is unclear, although it should be plain to anyone who actually understands the math and experimental setup.
An interesting experiment to consider is the delayed-choice quantum eraser experiment, in which a special detector detects which path a particle went through, and then the full results of the detector are carefully fully stomped over so that the particles of the detector (and everything else) are in the same exact state no matter which path had been detected. The configurations are able to interfere once this erasure step happens and not if the erasure step isn't done.
Another fun consequence of this all is that we can basically check what configurations count as the same to reality by seeing if you still get interference patterns in the results. You can have a setup where two particles 1 and 2 of the same kind have a chance to end up in locations A and B respectively or in locations B and A, and then run it a bunch of times and see if you get the interference patterns in the results you'd expect if the configurations were able to interfere. Successful experiments like this have been done with many kinds of particles including photons, subatomic particles, and atoms of a given element and isotope, implying that the individual particles of these kinds have no unique internal structure or tracked identity and are basically fungible.
An important thing to realize is that interference is a thing that happens between whole configurations of affected particles, not just between alternate versions of a single particle going through the slit.
https://en.wikipedia.org/wiki/Double-slit_experiment#Variati...