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: 
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> 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, 
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This is absolutely wrong.

I blame Schrodinger's cat thought experiment for this type of misunderstanding of how quantum mechanics experiments work.

There exist various physics frameworks, and in these frameworks there are elegant theories which have been tested over and over again and work beautifully within their framework. Classical mechanics works for macroscopic observations, classical electrodynamics for light, thermodynamics, an emergent theory from the many body problems of classical mechanics, also is a self contained framework etc. Quantum mechanics is the framework of the very small , a self consistent framework when the Heisenberg uncertainty has to be obeyed , with its own differential equations and mathematical formulations. It can be shown mathematically that it is the underlying framework for most other frameworks which emerge naturally from the laws of the microcosm.( except gravity, and most physicists hope that will be shown to be quantized in the future).

Paradoxes arise when one mixes frameworks without keeping track of the hypotheses entering the problem. Our measurements and measuring devices are in the classical frameworks. We explore the underlying quantum mechanical framework by using interactions that obey quantum mechanical rules and studying the result in our macroscopic detectors.

As an example, the whole experiment, poison and all, of Schrodinger's cat is a macroscopic detector of what happens at the quantum mechanical level. The quantum mechanical equations enter in calculating the probability of decay, and the dead or alive cat is the *measurement* of whether a decay happened or not. The "dead or alive" cat is as unknown as the "is there a measurement on the counter". The measurement will be there whether I look at it or not, if a decay happened, because measurement of counters is in the classical framework.

In the case of the double slit , any measurements, photos... are in the classical framework and will be there whether looked at or not. The quantum mechanical uncertainties enter at the level of the QM equations with the boundary conditions "particle impinging on two slits of certain width and distance' and "particle impinging on two slits of certain width and distance + a detector at one of the slits". 

The interference pattern observed ( or not) is in the probability distribution from the complex conjugate square of the wavefunction describing the experiment, the quantum mechanical framework. The measurement in the detecting devices is in the classical framework. 

P.S. an interesting experiment that shows origin of the difference in the two QM distributions [is here](https://phys.org/news/2011-01-which-way-detector-mystery-double-slit.html).
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.