Wheeler's delayed choice experiment
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Wheeler's delayed choice experiment was originally a thought experiment proposed by John Archibald Wheeler in 1978 (Mathematical Foundations of Quantum Theory, edited by A.R. Marlow, Academic Press). Wheeler proposes a variation of the famous Double-slit experiment of quantum physics, where the target detector can be changed at the last moment, according to a "delayed choice" of the observer. There is a detector screen, which will detect the normal wave interference pattern produced by the double slit, which can also be removed. Behind the screen are two tightly focused telescopes, aimed to observe one slit or the other, which would detect which path the photon traveled. According to the results of the double slit experiment, if we know which slit the photon goes through, we change the outcome of the experiment and the behavior of the photon. If we know which slit it goes through, the photon will behave as a particle. If we do not know which slit it goes through, the photon will behave as if it were a wave. This wave-particle duality of photons (and in fact all quantum particles) is one of the fundamental mysteries of quantum mechanics.
On the surface, the outcome of the experiment seems simple. If we observe the photons with the screen, we get the wave like behavior. If we observe the photons with the telescopes, we get the particle like behavior. However, imagine a more bizarre scenario where the scale of the experiment is magnified to astronomical dimensions: the photon has originated from a star or galaxy on the other side of the universe, and bent by an intervening galaxy, black hole, or other massive object, so that it arrives at our detector by one of two different paths. If we observe the photon with a screen like detector, such as a photographic plate or other imaging device (as in the original experiment), we should see an interference pattern, but if instead we use a binocular telescope (two telescopes focused to either side of the black hole) we will observe two small points, because the photons must behave like particles. So, in fact which side of the black hole did the photon go to? The photon has been traveling through space for billions of years, is it a particle or a wave?
[edit] Actual Experiments
In 2000,Yoon-Ho Kim, et al, reported success in their delayed choice quantum eraser experiment, a variation that combines Wheeler's delayed choice experiment with a quantum eraser experiment, so that the choice to observe the photon or not observe the photon is done after it hits our detector.
In 2007, Vincent Jacques, et al, reported an almost ideal experimental realization of Wheeler's delayed choice experiment. While previous delayed choice experiments had been reported, this is the first one doing so with Wheeler's original scheme of using a single-particle quantum state, as well as a relativistic separation between particle entry and choice (such that the choice could not be influenced by any unknown signal traveling at the speed of light). The 'choice' was made by a quantum random noise generator converted into an on/off signal which transmitted into either combining or not combining the two paths, all within 40ns, about one-fourth the 160ns that the pulse took to travel through the interferometer.
[edit] External links
[edit] References
- Vincent Jacques et al., Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment, Science Vol. 315. no. 5814, pp. 966 - 968 (2007).
