For interference to disappear, it is necessary that human experimentalists know which path the photon takes.

The statement you're referring to touches on a fundamental aspect of quantum mechanics, particularly in the context of wave-particle duality and interference phenomena, such as those observed in the double-slit experiment. In the double-slit experiment, when photons (or any quantum particles) are sent toward a barrier with two slits, they can exhibit interference patterns typical of waves if both slits are open and the path of the photons is not determined or measured. This results in a distribution of photons on a screen that demonstrates characteristic interference fringes. However, if an experimenter measures or determines which slit a photon goes through (which path information), the interference pattern disappears, and the photons behave like particles, producing a pattern that corresponds to the classical expectation of particles passing through two slits (with no interference). This phenomenon illustrates the principle of wavefunction collapse in quantum mechanics — the act of measurement influences the outcome of the experiment. In essence, for interference patterns to manifest, the experimental conditions must preserve the inherent quantum uncertainty regarding the paths taken by the photons. If the experimenter gains "which-path" information, it collapses the superposition of the photon's states, leading to the disappearance of interference.