# Deflating Delayed Choice Quantum Erasure

07 Jan 2016

The Delayed Choice Quantum Eraser is a widely misunderstood experiment. Apparently the common belief is that it's some kind of grand world-view-shattering mystery, with powers ranging from sending messages backwards in time (example pop-science article) to demonstrating the existence of conscious knowledge (example woo thread). Although there's continuous effort to correct these misconceptions, it's easy to run into them.

# Summary

1. You never see an interference pattern. The interference pattern only shows up when filtering after-the-fact, using the chooser's measurement results to group experimental runs.

2. Backwards-in-time effects aren't needed. Unless you insist on a false dichotomy that precludes entanglement being a thing. Common interpretations of quantum mechanics simply don't have that dichotomy. For example, in the Copenhagen interpretation, the observations are explained by the which-way photon's state being collapsed as soon as the test photon hits the screen (before the delayed choice).

3. Consciousness has nothing to do with this. The mathematical model simply makes no mention of anything besides the equipment. The experiment will have the same outcome whether or not a human is present.

(For example, consider that I just spent two thousand words explaining DCQE in detail, and the word "consciousness" only showed up in the context of common misconceptions. That wasn't on purpose. It just really isn't relevant.)

4. Most of the 'weird' is due to presentation. Ultimately all we're doing is either measuring the information needed to find the hidden interference patterns, or not. If you don't measure the needed information, you can't find the hidden patterns. Duh.

Jconnors - Mar 31, 2016
Isn't the big take away that the measurement (observer effect) isn't causing the collapse of the wave function, it is rather the possibility of having information or not having information that causes the collapse?
Craig Gidney - April 1, 2016
That is a useful insight to have, but I don't like to phrase it that way. I think people distort "possibility of having information" into wrong ideas like "Burning the paper the computer printed the result onto without looking at it takes away the possibility of having information, so the interference pattern should come back.".

One thing I want to try to communicate eventually, equivalent to what you're saying, is the concept of FOCUS ON THE COPIES. As long as an entangled copy of the information exists, no matter how obfuscated, there will be no self-interfering. Bringing back the self-interference requires un-making any copies. Un-making a copy is generally done by flawlessly running a copy-making process in reverse. Post-selection obscures the symmetry between making and un-making copies. Measurement is just thermodynamically-irreversible copy-making.
Alex Torres - Sep 25, 2016
Kim's version of DCQE has a very simple set up, it was designed to rule out completely any sort of physical decoherence/interaction with the only variable left being the availability or not of information.....there's no mathematical equation to explain that... .of course this is not a physical one since it belongs to the cognitive realm...that's the spookiness on this....besides that, nonlocality can very well explain the apparent retro causality...
Alex Torres - Sep 26, 2016
The video you are referring to is a layman's compilation of various top yt videos...so may I suggest a more explicit in terms of graphical cold explanation....then you might have a better grasp of why this experiment is so mind boggling....