Entanglement
Lesson 4 of 23 in the course “Quantum”. Free, with animations and a quiz.
A pair with one state for two
Entanglement is when two particles don’t have separate states, only one shared state. Example: a pair of qubits where each one alone is 50/50, but their answers always match. Measure the first and get 0. The second, even a thousand kilometres away, will also give 0. Each answer is random, but the link between them is rock-solid. Entanglement is a quantum computer’s main resource: without it, qubits would just be a pile of separate coins.
Maybe it’s just socks?
Physicist Reinhold Bertlmann always wore mismatched socks. See that the left one is pink and you instantly know the right one isn’t. No magic: he picked the colours in the morning, you just found out. In 1935 Einstein thought particles work the same way: the answers are built in beforehand, and quantum theory is simply incomplete. He called the instant link at a distance “spooky action at a distance”. Settling who was right needed a clever test. It came 30 years later.
The Bell test: nature versus socks
In 1964 John Bell designed a test. Two players each get a particle and a random question, and they can’t talk. If the answers are built in beforehand, like sock colours, the best possible win rate is 75% of rounds. With entangled particles it’s about 85%. Experiments kept showing more than 75%, and in 2015 a team in Delft closed the last loopholes. In 2022 Aspect, Clauser and Zeilinger won the Nobel for these experiments. Einstein was wrong: there are no answers built in beforehand.
A faster-than-light messenger? No
If the link is instant, can you send messages faster than light? No. Alice measures her particle and sees random 0s and 1s. Bob also sees random 0s and 1s. Alice can’t influence what comes up for Bob. The matches only show once they compare notes, and that needs an ordinary channel: a call, the internet. So entanglement doesn’t break Einstein’s speed-of-light rules. If a reel promises a “zero-delay quantum messenger”, it’s fake.
Entanglement through space
2017. The Chinese satellite Micius, orbiting at 500 km, sent entangled photons to two mountain stations in China, 1,203 km apart. The pairs kept their link, and the Bell test passed again. The same satellite was used for a Beijing–Vienna video call with an encryption key created the quantum way. That’s quantum key distribution: if someone eavesdrops, measuring leaves a trace, and the eavesdropping shows up. Banks and government networks in China and Europe are already testing such networks.
Check yourself
- Alice measured her particle and got 1. What will Bob get?
Correct answer: Also 1. The pair shares one state: “the answers match”. Distance doesn’t matter: Bob’s result will match Alice’s. - Can entanglement send a message instantly?
Correct answer: No: each side alone sees pure noise. Alice doesn’t control what comes up. The matches only show after comparing notes over an ordinary channel, which is no faster than light. - What did the Bell test experiments prove?
Correct answer: The answers aren’t built into the particles beforehand. With a pre-made “cheat sheet” the maximum is 75% wins. Entangled particles give about 85%, and experiments confirmed it. That’s what the 2022 Nobel was for.
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