The delayed-choice quantum eraser has confounded many people for well over two decades, now. During that time, its peculiar results have spawned some pretty far-fetched interpretations, including the notion that the experiment demonstrates so-called “retrocausality,” in which effects precede causes. Not true. It turns out that it’s really a sorting machine, not a time machine.

Much of the challenge in comprehending this experiment stems from the scarcity of operational details given on it, starting with the original paper by Kim et al. Vital specifics are often ignored or taken for granted, leaving the uninitiated to fend for themselves.

Another obstacle for many is understanding the nature of the entanglement that exists between the photons bouncing around between detectors in the experiment, as well as the degree of coherence they exhibit during these interactions. Entanglement is the hardest aspect of the experiment for most to swallow—even Einstein called it “spooky action at a distance.”

Modern quantum theory offers some helpful mathematical guidance for dealing with entanglement. The following YouTube video gives an excellent quantum mechanical summary of the delayed-choice quantum eraser: https://www.youtube.com/watch?v=SCdbMhQ8Wrk.

Entanglement, according to QM, kills interference, which accounts for why we don’t see fringes at the scanning detector D₀ in the experiment. Because of this “decoherence,” only a statistically mixed ensemble of the four possible wavefunctions exists at the signal detector D₀. To the eye (and to D₀), the combination of states looks like the clump pattern from two single slits. No interference fringes.

But coherence is not so much destroyed by entanglement as it is rendered inaccessible to direct local measurement. Consequently, to disentangle a pure wavefunction from the ensemble at D₀, you must combine each signal detection with its twin’s detection at one of the four idler detectors. It’s as if the pure wavefunction exists only “in spirit” at D₀., and to interact with it you must enlist the help of a “medium” in the form of correlated idler-twin detections. This is because the full wavefunction is actually shared by the entangled signal-idler photon pair, so it takes a series of correlated detections to get the full story.

Nonetheless, despite the “decoherence” from entanglement—and, indeed, because of it—the results of the delayed-choice quantum eraser can be accurately modeled using the metaphor of two quantumly entangled dice. In the paper presented above, I describe how such a model can reconstruct the symmetric and antisymmetric coincidence data of the original paper by Kim et al., thus providing a deeper insight into the stochastic nature of the experiment and contradicting those infamous “retrocausal” interpretations of the experiment.

To download the PDF of this paper click here or on the abstract page imaged above.

To download the PDF of my backgrounder on this famous experiment, click “Disentangling the Delayed-Choice Quantum Eraser.”