>[!abstract] >The quantum eraser experiment is a variation of the double-slit experiment that shows how the behavior of quantum particles depends not just on measurement but on whether which-path information is available in principle. When photons (or other particles) pass through a double slit, they form an interference pattern unless a detector records which slit they used, in which case the pattern disappears. In the quantum eraser setup, the which-path information can later be "erased" or scrambled, and the interference pattern reappears—even retroactively if the erasure happens after detection. The experiment illustrates the deep role of information, observation, and entanglement in quantum mechanics, challenging classical notions of causality and realism. >[!related] >- **North** (upstream): [[Double-slit experiment]] (the foundational setup demonstrating wave–particle duality) >- **West** (similar): [[Delayed-choice experiment]] (John Wheeler’s thought experiment where measurement choice seems to retroactively affect past behavior) >- **East** (different): [[Classical determinism]] (where outcomes are fixed, and measurement merely reveals pre-existing properties) >- **South** (downstream): [[Which-path information]] (the core detail: the availability or erasure of path information dictates interference)