The Wigner function is a kind of topographic map of probabilities: hills correspond to regions where a particle is most likely to be found, and valleys to where its appearance is practically ruled out. For a long time, negative 'valleys' on this map were considered an infallible marker that the system behaves quantum mechanically, obeying the laws of the microworld. However, as it turns out, classical light (that is, light behaving like ripples on a lake, without any quantum oddities) can reproduce these landscapes after clever filtering.

The key to the trick lies in selective event selection. The experimenters passed light through a threshold device, keeping only strong signals and completely ignoring weak ones. Imagine a public opinion poll where 'against' votes are methodically crossed out and 'for' votes are counted; the final picture naturally looks like unanimous agreement. Similarly, on the Wigner map, after such filtering, negative zones appear, even though the original light was perfectly ordinary. It is important to understand: these negative values are not real probabilities (after all, a probability cannot be less than zero) but a mathematical trick, akin to a negative balance in accounting, arising from the way data are processed.

This leads to an important conclusion: the negativity of the Wigner function does not guarantee the presence of quantum effects; a classical system, after signal selection, can deftly mimic quantum behavior. To avoid falling for this trick, researchers need more subtle tests that do not rely solely on visual inspection of the Wigner landscape. A deep understanding of this problem traces back to the work of Roy Glauber, who studied how light can masquerade as quantum, mimicking its characteristic statistical features.