Entangled photons are like two couriers marching in lockstep: their quantum states, particularly polarization, are synchronized so that measuring one instantly determines the state of the other, even at a distance. In a material, this synchronization gets disrupted. The nature of the disruption reveals what they encountered: a fork that scattered their routes, or a magnetic carousel that twisted their step. This is how the BELS method works (from Bell-state spectroscopy). It sends pairs of entangled photons through a sample and detects signal coincidences — correlations between photon detections by detectors. One effect, birefringence, splits the beam like a fork, altering polarization depending on the direction in the crystal. Another, Faraday rotation, rotates the plane of polarization under a magnetic field, like a magnetic carousel. Ordinary light cannot distinguish these effects: its chaotic mixture of polarizations hides subtle changes. But entangled light leaves an entropic trace — a unique pattern of correlations that encodes information about the encountered effects. This pattern is like a fingerprint, from which the sample's properties are reconstructed.

The method is based on the quantum mechanics of entanglement — a fundamental property where two or more particles form a single system with a shared wave function. For the experimental confirmation of Bell's inequalities violations and the development of quantum information science, Alain Aspect, John Clauser, and Anton Zeilinger received the Nobel Prize. BELS promises ultra-precise diagnostics for quantum devices: the method is sensitive to the slightest phase shifts and polarization rotations, so even a single defect in a nanchip, such as local strain or an impurity, becomes visible through changes in the correlation pattern. In the future, this could allow finding faults in quantum computers without direct intervention, by analyzing only the photons that pass through the circuit, preserving the integrity of the working device.