1. Diffraction of atoms and molecules
(a) Thomas Young's classic double-slit experiment is the simplest two-path interference device and was used by him in 1801 to establish the wave nature of light. A single slit creates cylindrical wavefronts that then pass through two other slits, positioned very close together, before creating an interference pattern on a projection screen. In 1909 Geoffrey Ingram Taylor used a similar set-up to prove that a weak light source that emits photons one at a time can still create an interference pattern. (b) The Talbot-Lau interferometer can be used to study the interference of atoms and molecules. It has an array of slits, each of which corresponds to the illuminating slit in the Young set-up. The second grating takes on the diffracting role of the double slit, while the third grating is a "mask". It has the same spacing as the interference pattern and allows the fringes to be identified. This set-up has also been used to show that interference disappears if the molecules collide with other gas particles (green circle) or if they emit photons (red line). This is the signature of decoherence.