Wheeler's delayed-choice gedanken experiment with a single atom

Abstract

The wave-particle dual nature of light and matter and the fact that the choice of measurement determines which one of these two seemingly incompatible behaviours we observe are examples of the counterintuitive features of quantum mechanics. They are illustrated by Wheeler's famous 'delayedchoice' experiment 1 , recently demonstrated in a single-photon experiment 2 . Here, we use a single ultracold metastable helium atom in a Mach-Zehnder interferometer to create an atomic analogue of Wheeler's original proposal. Our experiment confirms Bohr's view that it does not make sense to ascribe the wave or particle behaviour to a massive particle before the measurement takes place 1 . This result is encouraging for current work towards entanglement and Bell's theorem tests in macroscopic systems of massive particles 3 .The question of whether light behaves like a particle or wave had a long and strongly contested history until the advent of quantum mechanics, where it was accepted that it could indeed exhibit either behaviour. Conversely, it was de Broglie's hypothesis of matter waves 4 that deviated from the preceding view of massive bodies exclusively as particles, which was confirmed by the electron diffraction experiments of Davisson and Germer 5 . Even more bizarrely, the way in which an experiment is performed seems to induce one of these behaviours to the exclusion of the other. The question of whether a single photon in an interferometer passes through either one arm (as a particle) or both simultaneously (as a wave) led to Wheeler devising his famous gedanken experiment, which supposed that the decision of whether to attempt to measure particle or wave behaviour is made after the photon enters the interferometer. By removing the second beamsplitter of the interferometer (Fig. 1a), which-way information is revealed 6 , which precludes an interference measurement, while inserting the beamsplitter destroys information about the path taken by the photon and re-establishes a wave interference dependent on the phase difference φ between the arms.Although many experiments have shown particle-wave duality with photons 7 , including delayed-choice schemes 8-10 , delayed-choice quantum eraser experiments 11 and entanglement swapping using delayed choice 12 , only recently has the complete scheme proposed by Wheeler been realized experimentally 2 . By simultaneously ensuring that only a single photon is present in the interferometer at once, and that the decision of interferometer configuration is relativistically separated from the photon's entry to the interferometer, it was unambiguously shown that Wheeler's supposition that such a choice affects the 'past history' of the photon was correct.Recent advances in the trapping and cooling of atoms has led to the ability to readily observe wavelike phenomena with particles that have mass, such as the interference between two Bose-Einstein condensates 13 . However, progress towards demonstrating Wheeler's experiment with massive particles, such a QR NG b |0〉 |1〉 DLD φ π /2 π /2...

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