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Delayed Choice Quantum Eraser

Our choice affects how the particle acted in the past /


An experiment by Australian scientists has proven that what happens to particles in the past is only decided when they are observed and measured in the future.

Until such time, reality is just an abstraction.

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 and it’s 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.

The DCQE experimental setup:


Experimenter reproduces the interference pattern of Young’s double-slit experiment by shining photons at the double-slit interferometer.

After they are being fired it’s time for checking for an interference pattern at the detection screen.


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.


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.

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.

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 bizarre nature of reality as laid out by quantum theory has survived another test, with scientists performing a famous experiment and proving that reality does not exist until it is measured. Physicists have conducted John Wheeler’s delayed-choice thought experiment, which involves a moving object that is given the choice to act like a particle or a wave. The group reversed Wheeler’s original experiment, and used helium atoms scattered by light.

Common sense says the object is either wave-like or particle-like, independent of how we measure it. But quantum physics predicts that whether you observe wave like behavior (interference) or particle behavior (no interference) depends only on how it is actually measured at the end of its journey.

This is exactly what the ANU team have found:

If one chooses to believe that the atom really did take a particular path or paths then one has to accept that a future measurement is affecting the atom’s past, said Truscott. “The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behavior was brought into existence,” he said.

Associate Professor Andrew Truscott

ANU Research School of Physics and Engineering, Australian National University



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