# 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.

 « Quantum Teleportation's Two Subroutines Using Eigendecomposition to Convert Rotations and Interpolate Operations »