A Standard Quantum Mechanical Treatment to Rationalize the Delayed-Choice Quantum Eraser

The delayed-choice quantum eraser experiment has long been a source of intense debate, often interpreted as suggesting that a measurement performed in the present can retroactively influence the past behavior of a quantum system. This paper argues that such interpretations are based on a misunderstanding of quantum measurement and that the entire phenomenon can be explained using standard, forward-in-time quantum mechanics.

By modeling the experiment with an extended Mach-Zehnder interferometer, the authors demonstrate that the choice between 'which-path' detection and 'erasure' detection is equivalent to choosing between two mutually unbiased measurement bases. When the idler photon is measured in a basis that reveals which path the signal photon took, the signal photon's interference pattern is destroyed. When the idler photon is measured in a basis that 'erases' this information, the interference pattern is recovered. Crucially, this recovery is only visible when looking at specific coincidence counts between the signal and idler detectors, which acts as a filter for the data.

The authors map this process to a two-way Stern-Gerlach framework involving entangled spin-1/2 particles. This analogy clarifies that the 'erasure' is not a physical change to the past, but a selection process. Because the joint state of the entangled pair is fixed at the source, the correlations are established from the start. The delayed choice of the idler measurement basis merely determines which sub-ensemble of the signal data is being analyzed. By framing the experiment this way, the authors show that the illusion of retrocausality arises from asking illegitimate questions about the state of the signal photon before the idler measurement is complete.