In the two-slit experiment with single molecules, the "which-slit" measurement generally disrupts the interference pattern and reduces or eliminates the interference effects. This phenomenon is known as the quantum eraser effect.
In the traditional double-slit experiment, when particles such as electrons or photons pass through two slits, they can exhibit wave-like behavior and produce an interference pattern on a screen behind the slits. This interference pattern arises from the superposition of different paths the particles can take to reach the screen.
However, if a measurement is made to determine through which slit a particle passes, it introduces additional information about the particle's path. This measurement collapses the particle's wavefunction into a specific state, destroying the superposition and interfering with the formation of the interference pattern. The act of measurement disturbs the system and makes it behave more like classical particles with definite paths.
Interestingly, in certain variations of the two-slit experiment, researchers have demonstrated the quantum eraser effect. By implementing additional experimental setup and post-selection techniques, it is possible to "erase" the which-path information and restore the interference pattern. These methods involve manipulating the entanglement of the particle with another quantum system, and they demonstrate the non-local nature of quantum mechanics.
So, in summary, in the standard two-slit experiment, the "which-slit" measurement destroys the interference pattern. However, with more sophisticated experimental techniques, it is possible to recover the interference pattern in a process known as quantum erasure.