Playground · Quantum Eraser
Double-slit & quantum eraser
Fire particles one at a time at two slits. Each lands as a single dot — yet together they build an interference pattern, as if each went through both slits. Switch on a which-path detector and the pattern vanishes. Then erase that information and — impossibly — it comes back.
What's going on
With no which-path information, each particle behaves like a wave passing through both slits and interfering with itself — so probabilities follow envelope × cos² and you get bright and dark fringes. The moment you measure which slit it went through, that self-interference is destroyed and the fringes smear into a single smooth band. The astonishing part: if you erase the which-path record — so the information is fundamentally unavailable — the interference returns. It's not the "observer" or the detector's disturbance that matters, it's whether the which-path information exists anywhere. An illustrative model of a real, repeatedly-confirmed experiment.