Two coins that always match

Put two qubits through the right gates and they become entangled. Measure one and you get 0 or 1 at random. Measure the other, in the next room or the next galaxy, and it matches. Every time.

That alone is not strange. Put one left glove and one right glove in two boxes, mail them apart, open one: you know what is in the other. The gloves were decided when they were packed. Einstein said the qubits must be like the gloves, decided from the start, with the decision hidden from us.

John Bell, in 1964, found a way to tell gloves from qubits. If you can measure at more than one angle, and the results were fixed in advance, the pattern of matches is limited in a way you can count. Quantum mechanics predicts more matches than that limit allows. The test was run in 1972, 1982, 2015 and many times since, and it always comes out on the quantum side. The 2022 Nobel Prize was for these tests. There is no hidden decision. The coins pick when caught, and they pick together.

This does not send messages faster than light. You see random 0s and 1s at your end whatever the other person does. The match only shows up when you compare notes, and the notes travel the ordinary way.

What it is good for: entanglement is how qubits share their amplitudes, so that a turn on one path affects the whole computation. It is why the space of a quantum computer grows by doubling with each qubit added. Twenty qubits carry a million amplitudes; three hundred carry more than there are atoms in the known universe. You cannot read them out, but you can make them interfere.