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Science 16 February 2007:
Vol. 315. no. 5814, p. 909
DOI: 10.1126/science.315.5814.909b
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This Week in Science

Wave-particle duality is at the heart of quantum mechanics. Particles and photons can display both properties, and which property is measured depends on the type of measurement made. What if the experimental setup changes when the photon or particle is "in flight" and has already entered the experimental apparatus? Jacques et al. (p. 966) report an almost ideal realization of such a "delayed choice" experiment as formulated by Wheeler. A triggered single-photon source provides a mechanism for precise timing of the experiment within laboratory conditions. The behavior of the photon in the interferometer depends on the choice of the observable that is measured, even if that choice is made when the photon is already in the system.




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