What Is Quantum Entanglement and Is It Really Faster Than Light?

[NightHarbour30]

Quantum entanglement is a phenomenon where two particles become correlated in such a way that measuring one instantly affects what you will measure about the other, regardless of the distance between them. Einstein famously called it spooky action at a distance and spent years trying to prove it could be explained without such apparent non-locality. Bell's theorem in 1964 and subsequent experiments, most definitively Alain Aspect's 1982 tests and the loophole-free Bell tests of 2015, have confirmed that the correlations are real and cannot be explained by any local hidden variable theory. But is entanglement a genuine feature of physical reality???

The question of whether it allows faster-than-light communication is where the science gets important and where popular coverage frequently misleads. The answer is no, and the reason is subtle. When you measure an entangled particle and it collapses to a definite state, the distant particle simultaneously takes on a correlated state. But the outcome of your measurement is random. You cannot choose what result you get, which means you cannot encode information in it. Your colleague with the distant particle sees their measurement outcome is also random. Neither of you knows whether you have correlated results until you compare notes through a normal classical communication channel, which travels at or below the speed of light. The correlation is real. The information transfer is not.

Entanglement matters for quantum computing because it allows qubits to be correlated in ways that have no classical equivalent. Quantum algorithms can exploit entanglement to create relationships between qubits that let computation proceed across the entire entangled system simultaneously. It also matters for quantum communication, where entangled pairs can be used to distribute cryptographic keys in a way that is provably secure because any attempt to intercept the key destroys the entanglement and alerts both parties. China's quantum communication satellite network, the Micius programme, has demonstrated entanglement-based key distribution over thousands of kilometres.

[Ellie22]

The no-communication theorem is the piece that mainstream coverage almost always leaves out. Yes the correlation happens instantaneously. No you cannot use it to send information because you cannot control what result you get

[TheRizz00]

The Bell test experiments are genuinely one of the greatest achievements in experimental physics. The idea that you could test something as fundamental as whether local hidden variables exist and get a definitive answer is extraordinary. The answer being no is even more extraordinary

[ReplyGuy26]

China's quantum satellite network doing entanglement-based key distribution over continental distances is a genuine real-world application of entanglement that is not about computing at all. The security applications are closer to deployment than the computing applications

[Rogue Sam]

The confusion between entanglement and superposition is common and understandable. They are related but different. Superposition is about a single particle being in multiple states. Entanglement is about multiple particles being correlated in ways that exceed classical probability

[HitmanMatt53]

Einstein's discomfort with entanglement came from his conviction that physical reality should be local and realistic. The Bell experiments showed that at least one of those assumptions is wrong. We have not collectively worked out which one and that is an active philosophical debate

[Rachel93]

Quantum teleportation, which has been demonstrated over increasing distances including to satellites, uses entanglement to transfer quantum states between locations. It cannot teleport matter or information faster than light but it can perfectly copy a quantum state, which has uses for quantum networks

[Clever Georgia]

The fact that entanglement is destroyed by observation means quantum communication systems have an inherent eavesdropping detector built in. This is the security property that makes quantum key distribution genuinely different from classical encryption

[Sinead_47]

Einstein FTW. I know its over my head