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Quantum delayed choice experiement on wave/particle complementarity : advances in "Science"

From a naive perspective, quantum objects are often said to behave either as particles, i.e. localized in the spatial domain, or as waves, i.e. unlocalized and therefore capable of interfering.

A recent experiment showed however, without ambiguity, that it is necessary to get rid of this simple vision : the behaviour of quantum systems, such as photons, cannot be reduced to a binary, i.e. exclusive, classical description in terms of being either waves or particles.

Physicists from the Laboratoire de Physique de la Matière Condensée – LPMC (CNRS/UNS, see the team Quantum Information with Light & Matter (QILM)), du Laboratoire Matériaux et Phénomènes Quantiques – MPQ (CNRS/Univ. Paris Diderot) et de l’institut des Sciences Moléculaires d’Orsay – ISMO (CNRS/Univ. Paris Sud), have observed single photons being in states for which wave and particle aspects are superimposed in a controllable fashion. This property is shown in a "delayed choice" quantum experiment, which has been published in the prestigious journal Science Magazine.

Typical experiments on wave/particle complementarity consist of sending quantum objects, such as single photons, in an interferometer. The photons are first subjected to a first 50/50 beam-splitter. Then, if detectors are placed at each of the outputs of the beam-spliter, photons are detected in either of the two arms, having each a 50% probability. If, on the contrary, the two arms are recombined at a second beam-spillter to form an interferometer, interference patterns are observed at the output of the device, being the signature of a wave-like phenomenon.

The "tour de force" brought by the LPMC physicists is two-fold. On one hand, they built an interferometer which is open for one photon’s polarization orientation, and closed for the orthogonal orientation. On the other hand, they realized the experiment taking advantage of polarization entangled photons (so-called twin photons), i.e. showing non-local, or non-separable, quantum correlations. While one of the photons, say the "test photon", is sent to the interferometer, the second photon, say "the twin", is sent to another laboratory, 20 m away, using an optical fiber, which allows i) to separate in the time domain the measurements of the two photons in a space-like manner, and ii) to delay the choice of the measurement basis of the twin photon 20 ns after the test photon was detected, and therefore destroyed.

Then, measurements of given polarization states of the twin permitted the authors to infer the test photon to be submitted not only to either a closed interferometer (wave-like behaviour) or to an open interferometer (particle-like behaviour), but also to a superposition of both. In the latter case, the test photon is found in a wave-like and particle-like state superposition, with controllable weights. The reported measurements, in agreement with quantum physics predictions, indicate that one has to get rid of the classical and naive wave-particle duality description. Single photons are not either waves or particles, but rather irreducible quantum systems.

- Press-book related to this work (click on the links) :

- Some pictures related to this work : please have a look at the article Single photons in all their forms ! in the Galerie.