One of the most experimentally-tested consequences of quantum mechanics is the violation of Bell's inequalities, which provides evidence for the existence of entanglement and non-local correlations.
Bell's inequalities were derived by physicist John Bell in the 1960s as a way to test the predictions of classical physics against those of quantum mechanics. These inequalities are mathematical expressions that quantify the correlations between measurements made on entangled particles.
Entanglement is a phenomenon in which two or more particles become correlated in such a way that the state of one particle cannot be described independently of the others. When two entangled particles are measured, their measurement outcomes can exhibit correlations that are stronger than what can be explained by classical physics.
Experiments designed to test Bell's inequalities have been conducted since the 1970s, and the results consistently demonstrate violations of these inequalities. These experiments involve measuring properties of entangled particles, such as their spin or polarization, in different directions and comparing the correlations between the measurement outcomes.
The violation of Bell's inequalities suggests that the correlations observed in entangled systems cannot be explained by local hidden variables, which are hypothetical classical properties that determine the outcomes of measurements. Instead, it supports the quantum mechanical notion of non-locality, where measurements on one particle can instantaneously affect the state or measurement outcome of another particle, regardless of the distance between them.
The experimental confirmation of Bell's inequalities violations provides strong evidence for the unique and non-classical aspects of quantum mechanics, such as entanglement and non-local correlations, and highlights the departure of quantum phenomena from classical intuitions.