Why The Delayed Choice Quantum Eraser is NOT Retrocausal ft. @LookingGlassUniverse

Quick Overview

The Delayed Choice Quantum Eraser experiment does not demonstrate retrocausal influence; rather, the appearance of retrocausality is resolved by understanding that Alice's measurement outcome only sorts the pattern of her twin photon (Photon A) into two distinct double-slit patterns based on the path information recorded by Bob's delayed measurement of Photon B, meaning the final observed pattern is only determined after both measurements are compared.

Key Points: The experiment shows that if Bob performs the which-way measurement on Photon B, Alice's resulting pattern for Photon A becomes a single-slit pattern, confirming the which-way information collapses the interference pattern. If Bob performs the quantum eraser measurement (erasing the which-way information for Photon B), Alice's resulting pattern for Photon A reverts to a double-slit interference pattern, even if the erasure happens after Photon A hits the screen. The key to resolving the apparent paradox is that the final interference pattern for Alice's photons (Photon A) is only visible when Alice and Bob's data are correlated, specifically sorting Photon A's hits based on the outcome of Bob's delayed measurement on Photon B. The experiment, as set up by Kim et al. (2000), uses a BBO crystal to create entangled photon pairs; one photon (A) goes through the double-slit setup to a detector, and the twin (B) goes to a delayed which-way detector/eraser. When Bob's erasure measurement reveals which slit Photon B came from, the resulting pattern for Photon A is a single-slit pattern; when Bob erases the which-way information, Alice sees a double-slit pattern. The apparent retrocausality is resolved because the interference pattern is only visible after post-processing the data, meaning no information is sent backward in time to change the event at the screen. The demonstration uses polarization filters (Horizontal for Slit 1, Vertical for Slit 2) and a Calcite crystal to mimic the which-way measurement and subsequent erasure.