3. Disappearing fringes
Molecules that interact with their environment by colliding with other gas molecules or emitting thermal radiation can no longer create interference patterns. They lose their quantum behaviour because information about the molecules is now, in principle, available - even if an observer does not actually extract that information. These graphs show the loss of interference with carbon-70 molecules in a Talbot-Lau interferometer in terms of the "normalized visibility", which is a measure of the contrast between light and dark bands of the interference pattern. (a) If gas is added to the interferometer, the visibility drops exponentially as the gas pressure increases. (b) If the molecules are heated by a laser of increasing power, they get hotter and emit more photons, which causes the relative visibility to fall slowly but nonlinearly. The entanglement with the environment is mediated via the colliding molecules and thermally emitted photons, respectively.