A double-slit experiment without slits

The original double-slit experiment, performed 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 rather than a reproduction of the slits formed on the other side. The double-slit experiment and variations on it have since become a standard way to explore and teach wave–particle duality.

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

The researchers prepared a mixed beam of D₂ and He. The D₂ molecules, which are in a rovibrational 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 in the top left of the image) or a superposition of two orthogonal orientations (as shown in the top right).

The angle of the bond influenced the direction the molecule scattered off a He atom (green circle). At a 45° angle, the D₂ scattered in one direction; at −45°, it scattered in the other. But in a coherent superposition of both orientations, the D₂ scattering had two indistinguishable pathways and was thus expected to show an interference pattern, much like photons passing through slits.

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

The graph on the left shows the experimental scattering angular distribution for a situation analogous to a single-slit experiment (blue dots): a mix of D₂ molecules each in either one or the other orientation. A single slit doesn’t have interference (although mechanical slits cause diffraction). To verify that the double-slit scenario introduces interference, the researchers compared the results with those for a superposition state (red dots). The scattering patterns are clearly distinct, particularly around 90°. And the experimental data match well with calculations based on a single- and double-slit interpretation (black curves).

Some previous experiments have also used molecular scattering to perform double-slit analogues, but those studies relied on naturally occurring states to act as the slits, and many didn’t have a method to experimentally verify the presence of an interference pattern. Because Zare and his colleagues prepared the states they used, they can adapt their system to single and double slits. Their molecular double slit should apply beyond inelastic collisions to a broader class of bimolecular reactions. In future experiments, for example, the researchers plan to examine the hydrogen–deuterium exchange reaction H₂ + D₂ → 2HD. (H. Zhou et al., Science 374, 960, 2021 .)