/ Our choice affects how the particle acted in the past /
In quantum mechanics, the quantum eraser experiment is an interferometer experiment that demonstrates several fundamental aspects of quantum mechanics, including quantum entanglement and complementarity. A delayed choice quantum eraser experiment is first performed in early 1999, is an elaboration on the quantum eraser experiment that incorporates concepts considered in Wheeler’s delayed choice experiment (Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih, Marlan O. Scully). The experiment was designed to investigate peculiar consequences of the well-known double-slit experiment in quantum mechanics, as well as the consequences of quantum entanglement.
The emphasis is on a clear understanding of how the instantaneous “collapse” of the wave function due to a measurement at a specific time and place may be reinterpreted as a relativistically well-defined collapse over the entire path of the photon and over the entire transit time from slit to detector.
- First, the experimenter reproduces the interference pattern of Young’s double-slit experiment by shining photons at the double-slit interferometer and checking for an interference pattern at the detection screen.
- Next, the experimenter marks through which slit each photon went and demonstrates that thereafter the interference pattern is destroyed. This stage indicates that it is the existence of the “which-path” information that causes the destruction of the interference pattern.
- Third, the “which-path” information is “erased“, whereupon the interference pattern is recovered. (Rather than removing or reversing any changes introduced into the photon or its path, these experiments typically produce another change that obscures the markings earlier produced.)
If a photon manifests itself as though it had come by a single path to the detector, then “common sense” (which Wheeler and others challenge) says it must have entered the double-slit device as a particle. If a photon manifests itself as though it had come by two indistinguishable paths, then it must have entered the double-slit device as a wave. If the experimental apparatus is changed while the photon is in mid‑flight, then the photon should reverse its original “decision” as to whether to be a wave or a particle.
Wheeler pointed out that when these assumptions are applied to a device of interstellar dimensions, a last-minute decision made on Earth on how to observe a photon could alter a decision made millions or even billions of years ago.
The quantum eraser is a clever experimental design, in which first a quantum system, prepared to be in a superposition of states corresponding to various values of an observable O, undergoes a “measurement” by getting entangled with another microscopic system playing the role of “measuring apparatus,” so that the value of O gets correlated with the value of an observable M of the “apparatus.”
Then this correlation gets destroyed by performing on the “apparatus” the measurement of an observable incompatible with M.