Classical Light Masters 'Quantum' Tricks ⚡ экспресс
Negative values of the Wigner function (a quasiprobability distribution) are traditionally seen as a hallmark of quantumness. However, new work shows that a classical model of squeezed light with postselection (selecting events based on an amplitude threshold) reproduces these negativities for photon-added states. Using a classical model of homodyne detection and tomography, they obtained negative Wigner values for vacuum and weak coherent states with an added photon. It's like throwing away the 'wrong' frames of a film creates the illusion of levitation—classical physics masquerading as a quantum effect.
The Wigner distribution is like a topographic map of probabilities: hills where the particle is most likely, and valleys where it shouldn't be. For a long time, negative 'valleys' were considered a foolproof sign that the system behaves quantumly. Yet classical light, like ripples on a lake, can reproduce these terrains after some clever filtering.
The secret lies in event selection: by passing light through a threshold, scientists kept only strong signals, ignoring the weak ones. It's exactly like a survey where all the 'no' answers are discarded and only 'yes' answers are kept, making the outcome seem one-sided. Thus, negative zones appear on the map, even though the original light was ordinary. The negative values themselves aren't real probabilities, but a mathematical trick, like a negative balance in accounting.
This means: Wigner negativity doesn't guarantee the presence of quantum effects. To avoid being fooled, more refined tests are needed. This insight dates back to Roy Glauber, who studied how light can disguise itself as quantum.
🎯 In 1932, Eugene Wigner devised a way to describe quantum particles using the language of probabilities, but with a catch: negative values in his formula are not real probabilities, but a calculational trick. That's why the distribution is called a 'quasi-distribution'.