Quantum Eraser
Add a which-path detector to the double slit and the interference pattern vanishes — the photon behaves like a particle. Add an eraser that destroys the path information and the pattern returns. The experiment reveals that it is the availability of information, not any physical disturbance, that determines whether quantum interference occurs.
The double-slit and the mystery of interference
In the classic double-slit experiment, a photon (or electron, or molecule) passes through two slits simultaneously as a wave, producing an interference pattern of bright and dark fringes on a screen. Each photon, sent one at a time, still interferes with itself — it has no definite trajectory, only a probability amplitude that threads through both slits. The interference is between the probability amplitudes of the two paths, not between two different photons.
Which-path information destroys interference
If you attach a detector to one or both slits that can tell you which slit the photon went through — even in principle, even if you never look at the detector — the interference pattern vanishes. The photon now has a definite trajectory and behaves like a classical particle, producing two blobs on the screen instead of fringes. This is Bohr’s complementarity principle: you cannot simultaneously know the wave property (interference fringes) and the particle property (which path). The key insight is that “observation” does not require a conscious observer; it requires only that the path information become encoded in the environment in principle.
The eraser: destroying information restores interference
The quantum eraser, proposed by Scully and Drühl in 1982, adds a twist: after the photon passes through the slits with detectors attached, an “eraser” can destroy the which-path information before it is ever read. When this happens — even if the erasure occurs after the photon has already hit the screen in the delayed-choice version — the interference fringes return, but only when you select the subset of photons whose path information was erased (the “coincidence” subensemble). The pattern never violates causality: you cannot use the eraser to send information backwards in time because you need a classical channel to identify which subset to look at.
What it means: observation without a conscious observer
The quantum eraser makes vivid that the “observer effect” in quantum mechanics has nothing to do with minds or consciousness. What matters is entanglement with the environment: if the photon’s state becomes correlated with any other system in a way that encodes path information, the coherence responsible for interference is lost. This process — decoherence — is why quantum superpositions are so fragile at macroscopic scales, and why the boundary between quantum and classical behaviour is really a boundary between isolated and entangled systems.