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Einstein-Bohr Gedanken Experiment Performed at the Molecular Level

Tuesday, December 2, 2014

SOLEIL Synchrotron, France: Catalin MIRON | | +33 6 73 43 05 14
KTH, Sweden: Faris GEL’MUKHANOV |
Tohoku University, Japan: Kiyoshi UEDA |

French, Swedish and Japanese teams managed to realize, for the first time, a molecular photoionization experiment illustrating a gedanken experiment first proposed by Albert Einstein and Niels Bohr in their discussion on whether the elementary particles constituting our surrounding world should be seen as particles or waves. Challenging the emerging understanding of quantum physics, this original experiment continues one of the richest public debates in the history of Science, which has already led to several Nobel Prizes, including the one awarded in 2012 to Serge Haroche. Their result is published online on Nature Photonics the 1st December 2014.

Figure1: Schematic representation of the double slit gedanken experiment born from the debate of Einstein and Bohr, with coupled (a) and decoupled (c) massive slits, and the schematics of the materialization of this thought experiment using a molecular photoionization experiment where the two slits are replaced by two coupled (b) and decoupled (d) oxygen atoms.

The birth of quantum mechanics revolutionized the vision that scientists had over the world surrounding them. This revolution engendered passionate scientific and philosophical discussions between them, some of which are still alive today. One of the most important ones, concerning the complementarity principle (particle-wave duality of quantum objects), involved two well-known physicists, Albert Einstein and Niels Bohr. Einstein, at this early age of quantum mechanics, challenged the complementarity principle by suggesting a gedanken experiment, involving a double slit experiment with a moving macroscopic slit. The famous Young’s double slit experiment illustrates the wave nature of light with an interference pattern observable on a screen behind two slits. A gedanken experiment is an ideal thought experiment to test hypothesis or theory and to evaluate their consequences. However it may or may not be possible to physically realize such an experiment. From their animated discussions this conceptual experiment evolved and became the famous «Einstein-Bohr recoiling double-slit gedanken experiment», in which the momentum transfer between a particle (photon) and a recoiling slit, could allow identification of which slit the photon passed through on its way to the screen, thus destroying the interference pattern. Unfortunately, the weight of a massive, macroscopic slit makes this kind of measurement impossible.

80 years after, a French (PLEIADES beamline at SOLEIL synchrotron) and a Swedish team (Royal Institute of Technology), with the contribution of a Japanese researcher (Tohoku University), have realized this gedanken experiment at the molecular level. They replaced the double slit by the biatomic oxygen molecule, in which each atom now plays the role of a slit. In the experiment the neutral molecule is excited with soft x-ray synchrotron radiation into an unstable electronic state where the molecule dissociates into two atoms that are quickly moving apart. Subsequent de-excitation of the system proceeds through the ejection of a fast (Auger) electron. Using a state-of-the-art electron-ion coincidence experiment, the scientists were able to directly measure the momentum exchange between the emitted Auger electron and the molecular or the atomic ion, playing the role of ultra-light microscopic slits. Two possibilities arise. In the first one, an electron is ejected soon after the excitation, before the molecule has time to dissociate, and the bond between the two oxygen atoms is still strong. The recoiling momentum will thus be the same for both atoms (the two slits are attached) making it impossible to determine from which atom the electron was ejected, interferences thus being observed. In the second one, the Auger electron is ejected later, when the molecule has begun to dissociate, and the emitted electron transfers the recoil momentum to only one of the two oxygen atoms (see figures). This asymmetric momentum transfer distinguishes the «path» (which slit the electron emerged from) and thus quenches the interference pattern.

Figure2: Experimental demonstration (left panels) and theoretical simulation (right panels) of the materialization at the molecular level of the recoiling double slit gedanken experiment. An interference pattern similar to that observed in the famous Young’s double slit experiment is observed in the case when the site (slit) from which the electron has been emitted is not identifiable (upper panels), while this pattern is washed out when the asymmetric momentum transfer (Doppler shift) between the emitted electron and the oxygen ion left behind allows identification of the emission site (lower panels).

To conclude, scientists managed for the first time to materialize a moving slit experiment first proposed 80 years ago as a gedanken experiment by Bohr and Einstein by using x-ray photoemission from molecular oxygen. While their results agree with the vision of Niels Bohr, nevertheless they also demonstrate that Albert Einstein was right when he wrote: «Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world [...].»

Reference of the publication:
«Einstein–Bohr recoiling double-slit gedanken experiment performed at the molecular level»
Xiao-Jing Liu, QuanMiao, Faris Gel’mukhanov, Minna Patanen, Oksana Travnikova, Christophe Nicolas, Hans Ågren, Kiyoshi Ueda and Catalin Miron.
Nature Photonics 2014, Published online 1st Dec. 2014
DOI: 10.1038/NPHOTON.2014.289

SOLEIL is the French national synchrotron radiation facility, a multi-disciplinary instrument and research laboratory. Located in Saint-Aubin (30 kms South of Paris), SOLEIL it is a very powerful light source used to explore all states of matter. Synchrotron radiation is provided by high energy electrons accelerated almost at the speed of light, in an accelerator (storage ring) with a circumference of 354 meters. Collected in different points around the ring, this radiation is directed to outputs, the SOLEIL beamlines. Each beamline is a laboratory by its own, being equipped to prepare and characterize the samples under study and to analyze the collected data. SOLEIL has already contributed to significant advances in fundamental and applied research, in a broad variety of fields from Medicine and Biology to Pharmacy, Geophysics, Chemistry, Physics, new and ancient Materials, Electronics, Archaeology...