The correlation of particles separated by distance so that what happens to one affects the other. For decades, quantum entanglement has been a major topic in quantum mechanics. It has always been a point of contention with scientists in classical physics because it flies in the face of known behaviors. See
quantum mechanics.
How Particles Are Entangled
Using various methods, including magnets and lasers, both electrons and photons can be entangled. For example, it might take a billion laser pulses to generate one entangled pair. When an entangled pair is created, one can be sent a long distance while retaining its relationship with the other.
For example, entangled qubits are used in quantum cryptography whereby if one of the particle pairs is tampered with, the other is affected. This entanglement relationship is also used to form complex logic gates in quantum computers.
Spooky Action
Albert Einstein called quantum entanglement "spooky action at a distance." See
quantum computing.