Jennifer Chu in Phys.org:
Quantum entanglement may appear to be closer to science fiction than anything in our physical reality. But according to the laws of quantum mechanics—a branch of physics that describes the world at the scale of atoms and subatomic particles—quantum entanglement, which Einstein once skeptically viewed as "spooky action at a distance," is, in fact, real.
Imagine two specks of dust at opposite ends of the universe, separated by several billion light years. Quantum theory predicts that, regardless of the vast distance separating them, these two particles can be entangled. That is, any measurement made on one will instantaneously convey information about the outcome of a future measurement on its partner. In that case, the outcomes of measurements on each member of the pair can become highly correlated.
If, instead, the universe behaves as Einstein imagined it should—with particles having their own, definite properties prior to measurement, and with local causes only capable of yielding local effects—then there should be an upper limit to the degree to which measurements on each member of the pair of particles could be correlated. Physicist John Bell quantified that upper limit, now known as "Bell's inequality," more than 50 years ago.
In numerous previous experiments, physicists have observed correlations between particles in excess of the limit set by Bell's inequality, which suggests that they are indeed entangled, just as predicted by quantum theory. But each such test has been subject to various "loopholes," scenarios that might account for the observed correlations even if the world were not governed by quantum mechanics.