It's the TEM 01 beam structure that allowed the simultaneous measurement of the wave-like interference pattern and the particle-like path through one of the slits.

His analyses showed that experimentally observed interference patterns, maxima and minima, could in fact be explained in terms of bright (detectable) and dark (undetectable) states of light.

Classical physics theories suggest that when two or more electromagnetic waves interfere destructively (i.e., with their electric fields canceling each other out), they cannot interact with matter.