A double slit experiment without a slit

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The original double slit experiment, carried out by Thomas Young in 1801, demonstrated the wave nature of light. When the light – and in later experiments, electrons, atoms, and molecules – passed through two mechanical slits, an interference pattern formed on the other side, rather than a reproduction of the slits. The double slit experiment and variations thereof have since become a standard method for exploring and teaching wave-particle duality.

Now Richard Zare, Nandini Mukherjee and their colleagues from Stanford University have a molecular deuterium (D2) and helium scattering experiment analogous to a double slit. The results show the influence of quantum overlay on molecular scattering.

The researchers prepared a mixed jet of D. before2 and he. The D2 Molecules that are in a vibrational state then inelastically collided with He atoms and relaxed into their rotational ground state. The bond axis between the two D atoms can have different orientations relative to the beam axis (as shown above on the left in the picture) or a superposition of two orthogonal orientations (as shown above on the right).

The bond angle influenced the direction in which the molecule was scattered on a He atom (green circle). At a 45 ° angle, the D is2 scattered in one direction; at -45 ° it scattered into the other. But in a coherent superposition of both orientations, the D2 Scattering had two indistinguishable paths and was therefore expected to show an interference pattern, similar to photons passing through slits.

Source: H. Zhou et al., science 374, 960 (2021)

A sequence of a pump laser pulse followed by a Stokes laser pulse puts the molecules in one of three states – oriented at 45 °, at −45 ° or in a coherent superposition of both 45 ° and −45 ° – with the bond orientation through the polarizations . According to D2 and He collided, Zare and his colleagues measured the scattering angle with an ionizing laser and a mass spectrometer.

The graphic on the left shows the experimental scattering angle distribution for a situation analogous to a single-slit experiment (blue dots): a mixture of D2 Molecules in one or the other orientation. A single slit has no interference (although mechanical slits cause diffraction). To verify that the double-slit scenario introduces interference, the researchers compared the results with those for an overlay state (red dots). The scattering patterns are clearly pronounced, especially around 90 °. And the experimental data agree well with calculations based on single and double slit interpretation (black curves).

Some previous experiments have also used molecular scattering to perform double-slit analogs, but these studies relied on naturally occurring states to act as slits, and many did not have a method to experimentally verify the presence of an interference pattern. Since Zare and his colleagues have prepared the states used, they can adapt their system to single and double columns. Beyond inelastic collisions, their molecular double gap should apply to a broader class of bimolecular reactions. In future experiments, for example, the researchers want the hydrogen-deuterium exchange reaction H. investigate2 + D2 → 2HD. (H. Zhou et al., science 374, 960, 2021.)


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