Here a non technical view, intro here, more at the link:
What Is Quantum Entanglement? Skip the heady and abstract physics lectures. Let’s talk about socks By DAN GARISTO in IEEE Spectrum
When pushed to explain why quantum computers can outspeed classical computers, stories about quantum computing often invoke a mysterious property called “entanglement.” Qubits, the reader is assured, can somehow be quantum mechanically entangled such that they depend on one another. If more detail is needed, the reader is told that entanglement links qubits no matter how far apart they are—so long as the qubits are “coherent.”
For the reader, things are far from coherent. Sure, entanglement is an important aspect of quantum computing. But what exactly is it?
In a few words, entanglement is when multiple objects—such as a pair of electrons or photons—share a single quantum state. Like threads in a tangle of yarn, entangled objects cannot be described as independent entities.
That explanation might be poetic, but it shouldn’t be satisfying. Things are not so simple or concrete. But with a little bit of high-school-level math (near the end of this story), our intuitions—based on a lifetime of classical physics—can be retrained and redirected just a bit.
However, we should also make the following disclaimer: No brief explanation can be expected to convey a comprehensive understanding of quantum mechanics. Our goal is simply to illustrate the basic concepts behind entanglement, so the reader can gain a more thorough understanding of what’s actually going on in this foundational phenomenon behind quantum computing.
Let’s begin with a slightly modified example from the celebrated Northern Irish physicist John Stewart Bell: .... '
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