https://phys.org/news/2018-06-tracking-mystery-entangled-particles.html

Entanglement is one of the most impressive quantum physical phenomena. It describes the property of two particles not behaving like two independent objects, but like a single physical object. The entanglement is not to be understood spatially: Entangled particles correlate with each other in terms of their properties. This means that if you change the properties of one particle, the other particle changes at the same time, no matter where it is. Particles of light (photons) can be entangled by splitting a single particle into two photons in a laser arrangement with a special crystal. In optics, entangled photons are a major component in the development of new quantum measurement methods. They can be used because the measurement capacity of an entangled photon pair is larger than the one of two individual photons. However, quantum entanglement leads to the observation of relationships between measurements at the photon pairs, which can only be explained quantum-mechanically and not with concepts of classical physics.

Until now there has been no method to produce photon pairs that do not show quantum mechanical, but only classical energy correlations. In an experiment, a research team of the Institute of Applied Physics at the University of Bern has now succeeded in transforming the observed correlations of photon pairs from purely quantum-mechanical to completely classical. This transition represents a novelty, since quantum and classical correlations are difficult to reconcile. The researchers were able to demonstrate the transition in an experiment with a new method in which they were able to control the correlation of the energies of two photons. The results were published in the journal Nature Communications Physics.