A quantum interference effect where measurement erases which-path information, restoring wave-like behavior.The quantum eraser is a phenomenon in quantum mechanics where the act of 'erasing' which-path information about a particle restores its wave-like interference pattern. In a classic double-slit experiment, if you measure which slit a photon passes through, the interference pattern disappears — the photon behaves like a particle. But if you then destroy or 'erase' that which-path information (even after the photon has been detected), the interference pattern can reappear. This demonstrates that the observer's knowledge, not just the measurement itself, plays a role in determining the outcome — a counterintuitive cornerstone of quantum theory.
The quantum eraser was proposed by Marlan Scully and Kai Drühl in 1982, building on ideas from the Bohr–Einstein debates. The first experimental realization was by Yoon-Ho Kim and collaborators in 1999, using spontaneous parametric down-conversion to create entangled photons.
The experiment typically uses entangled photon pairs. One photon goes to a screen, the other to a detector that reveals which slit its partner took. Because of entanglement, measuring the partner's path destroys interference on the screen. However, if the partner's which-path information is erased — for example, by sending it through a beam splitter that mixes the paths — the screen photon's interference pattern is restored. The key is that the erasure happens after the screen photon has already been detected, yet the pattern still changes, highlighting the non-local nature of quantum correlations.
While largely a foundational experiment, quantum eraser principles inform quantum cryptography and quantum imaging, where controlling which-path information can enhance resolution or security.
In a quantum eraser experiment, the decision to erase information can be made after the photon has already hit the screen — yet the interference pattern still changes retroactively, a fact Einstein famously disliked.
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