Albert Einstein colorfully rejected the idea of quantum entanglement as "spooky-action-at-a-distance."
In layman's terms, quantum entanglement is the ability of distributed particles to share a state of quantum superposition, to be precise.
Doesn't it sound spooky? Maybe we should refresh the notion of superposition.
Particles have a spin. Up or down. The direction of the spin is not determined until you measure it. But once you measure it, it will instantly collapse to either one spin direction for you to observe. This is the superposition of a single particle.
Quantum entanglement says two particles can share a state of superposition. Their spins correlate. Once you measure one particle's spin, the state of the other particle changes instantly.
Doesn't it sound spooky? Maybe we should talk about scales.
When we say the two particles are distributed, they can be direct neighbors within the same atom. They can be a few feet away from each other. But they can also be light-years apart. It doesn't matter!
When we say the state of the particle changed instantly, we mean instantly. Not after a few seconds. Not after a tiny fraction of a second. But instantly.
The two particles can be light-years away from each other, yet when we measure one of them, the other changes its state in the exact moment.
Sounds spooky, right?
"But how do we know?"
We have not tested such a setting with particles light-years away. But we know the underlying math.
Long before the first experiment provided evidence, a group of geniuses developed formulae that predicted how an entangled pair of particles would behave. Einstein was one of them. And while he understood the language of math like no one else could (very few could…maybe), he didn't like what math told him this time.