Measuring the quantum W state: Seeing a trio of entangled photons in one go

The concept of quantum entanglement is emblematic of the gap between classical and quantum physics. Referring to a situation in which it is impossible to describe the physics of each photon separately, this key characteristic of quantum mechanics defies the classical expectation that each particle should have a reality of its own, which gravely concerned Einstein.

source https://phys.org/news/2025-09-quantum-state-trio-entangled-photons.html