Question: A gentleman on youtube called Tom Campbel, PhD, claims the following. If we perform the double slit experiment with 2 kinds of detectors, one which records the *which path* information about the particle and another that detects the pattern formed by the particles in the screen, something very unusual would happen:

If we looked at the screen detector alone, we would see a clump pattern, because there exists data, information, on the *which path* part of the experiment.

If we looked at the *which path* device alone, it would show data regarding which path each particle took.

If we looked at none, the result would remain uncertain.

If we would let the *which path* recording device sealed away for 100 years, never examining it's data, and in the mean time we'd never look at the screen pattern device, and if, 100 years later, we were to completely destroy the information recorded in the *which path* device so it could never, ever be recovered, when we looked at the daata f the screen pattern it would show an interference pattern, instead of a clump pattern, even though we tood the information on *which path*. but since we completely and utterly erased that information without first observing it, the *which way* measurement would be like it never existed in the first place, because there isn't data anywhere in the universe of what it recorded.

Is there any truth to this?

Could any of you conduct such an experiment (no need for the 100 years, but absolute need for the complete destruction of the information in the *which path* measurement device. Is that even possible? to completely destroy information?)

Answer: 
In short, hiding the data is no different than seeing the data.

**Quantum limits how good our meters are but we can always build a bad meter**

An "observation" happens any time a Q (a "small" quantum system) interacts with a C (a "large" classical system) enough to perturb the Q. In the double slit the Q is the photon, and the C's are any detector device. If a C gives us some information, it must count as a measurement, so any C that can figure out which slit the photon passed through will destroy the interference pattern. However, we always do worse: a C that erases it's own data after a set amount of time will still count as an observation (destroy the pattern) even it said device is worthless to us. Even putting a brick in front of both slits counts as "observing" which slit each photon went through!

**No time travel/faster than light**
We can't build a C that will allow someone i.e. a lightyear away and a year in the future to choose whether or not the photons going through the slit are observed. It is "observed" whether or not they intercept the i.e beams that tell them the results.
The question in focus here is known as that of "quantum erasing" and relates to the big interpretation issues of quantum mechanics. One may look e.g. at the Wikipedia article: <https://en.wikipedia.org/wiki/Quantum_eraser_experiment> or at Preskill's lectures notes [http://www.theory.caltech.edu/~preskill/ph219/chap2\_15.pdf](http://www.theory.caltech.edu/%7Epreskill/ph219/chap2_15.pdf) (section 2.5.4).
Basically, the answer to your question ("is there any truth to this?") is yes, although much depends on what one accepts as a bona fide "recording" of the data. If it means intrication with a quantum object (here both for the which path device and the screen), then it can indeed be "erased" and interference recovered. If recording means something like a quantum measurement implying Born postulate ("wave packet collapse" involving a "macroscopic apparatus") then it cannot be erased and interference will not be recovered. What makes the difference (and is there really one) between a quantum object and a "macroscopic apparatus" has been a hot question for ages. On practical grounds, interaction with the environment will cause "decoherence" making "recording" look very much like "measurement", but this leaves open Bell's criticism about "For All Practical Purpose" arguments.