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Quantum eraser: A proposed photon correlation experiment concerning observation and "delayed choice" in quantum mechanics. M. O. Scully and K. Drühl in Phys. Rev. A 25:2208 (1982).  What the paper says!?

The paper aims to find a situation in which

«a reasonable quantum mechanical analog of an observer may be woven into calculations associated with some envisioned experimental setup.»

This is a beautiful paper, with a great idea, great results and exquisitely written ({{onlinequote|«to prepare the reader for the arguments which follow, we summarize our results in the next few lines.») It requires utmost attention.