Entangled photons are like two messengers walking in lockstep. Inside a material, their synchrony is broken. The way it breaks tells you what they met: a fork that scattered their routes, or a magnetic carousel that spun their step. That's how the BELS method works. It sends pairs through a sample and detects coincidences in the signals. One effect (birefringence) splits the beam like a fork; the other (Faraday rotation) twists the polarization like a magnetic carousel. Ordinary light can't tell them apart, but entangled light leaves an entropic trail — a unique pattern of correlations.

The method is based on the quantum mechanics of entanglement, for which Alain Aspect, John Clauser, and Anton Zeilinger won the Nobel Prize. BELS promises ultra-precise diagnostics for quantum devices: even a single defect in a nanochip will become noticeable. In the future, this will allow finding faults in quantum computers without direct intervention.