Picture a pair of gloves. One goes in each of two boxes, and the boxes travel to different countries. Opening one tells you what is in the other. The gloves get one thing wrong. They were a left and a right before anyone looked. Entangled objects are not: neither holds an answer until it is measured.
Entanglement is sold today as a way to protect a communication line. Anyone listening in disturbs what is sent, so the intrusion shows up. Governments and banks buy it for a few dedicated links.
Now the part that matters: it is slow, and it protects the line only. In 2020, a Chinese satellite shared a secret key between two ground stations 1,120 kilometres apart, at one eighth of a bit per second. That is a rate for keys, not for traffic. So when someone offers an unhackable quantum internet, ask what is protected. The line is. The computers at each end are not.
One mistake comes up constantly. Entanglement does not carry messages faster than light. To send anything you would have to choose the result you get, and you cannot: your own result is random. The two sides find out that their results match by comparing notes over an ordinary phone line.
Inside a machine, entanglement is now routine engineering. Over distance the record links need a satellite or a dedicated cable, and carry a trickle. Einstein raised entanglement in 1935 as an objection to quantum theory. It is now a line in a purchase order.
Qubit · Superposition · Quantum computer · Quantum mechanics
Two objects are prepared so that they share one state. Measuring one decides what the other will give, at any distance. Neither had a definite value beforehand, which is what separates this from an ordinary coincidence.
Pairs of particles of light are the standard laboratory case, and a Chinese satellite shared such pairs between ground stations more than a thousand kilometres apart. Inside a quantum computer, entangled qubits are created every time a program runs.
The match between the two results appears instantly, and no information travels. Since each result is random, nothing can be sent by choosing it, so nothing overtakes the speed of light.
No. Entanglement needs objects that are kept completely undisturbed. A human body exchanges heat and light with its surroundings without stopping, and that contact destroys the link far faster than anything could use it.
Superposition describes one object holding several states at once. Entanglement describes two objects sharing one state, so that their results always match. Entanglement needs superposition; the reverse is not true.
Yes. Laboratories have confirmed it since 1972, and the experiments of 2015 closed the last objections. The 2022 Nobel Prize in Physics was awarded for that work.
1. Einstein, A., Podolsky, B. & Rosen, N., Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?, Physical Review, 1935.
2. The Nobel Prize in Physics 2022, awarded to Alain Aspect, John F. Clauser and Anton Zeilinger, NobelPrize.org, 4 October 2022.
3. Yin, J. et al., Entanglement-based secure quantum cryptography over 1,120 kilometres, Nature 582, 501-505, 18 June 2020, doi 10.1038/s41586-020-2401-y (key rate 0.12 bit per second).
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