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The First Universal Quantum Network
MrSeb writes "German scientists at the Max Planck Institute of Quantum Optics have created the first 'universal quantum network' that could be feasibly scaled up to become a quantum internet. So far their quantum network only spans two labs spaced 21 meters apart, but the scientists stress that longer distances and multiple nodes are possible. The network's construction is ingenious: Each node is represented by a single rubidium atom, trapped inside a reflective optical cavity. These atoms communicate with each other by emitting a single photon over an optical fiber. Each atom is a quantum bit — a qubit — and the polarization of the photon emitted carries the quantum state of the qubit. The receiving qubit absorbs the photon and takes on the quantum state of the transmitter. Voila: A network of qubits that can send, receive, and store quantum information. In another, probably more exciting test, the emitted photons were actually used to entangle the rubidium atoms."
You're wrong. Quantum entanglement does not allow any information to be transferred faster than light.
Sitting a million miles away from your partner with your entangled particle, the only thing you know is that you and your distant partner will measure a correlated result from that particle -- a fact you already knew a million years ago when you parted company in your very-nearly-light-speed ship.
You do not know, and can not control, what the value will be. You do not know if the other person has measured their particle's state or not. Measuring the state destroys the entanglement. All you know after is that the result you got will be correlated with what they get, or got.
No information transfer is possible.
However entanglement is useful for other things. Like networks where you can detect if someone snooped on your packets.
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When you change the state of one, it changes the state of another. Why could you not just view the state as a way of transferring information?
Because you can't control the state that it collapses to when you measure it and break the entanglement. You can't tell whether or not the person on the other end has already done this. All you know is that whatever state you measure, they will see a correlated result. Which you already knew; you've learned nothing.
A useful analogy* -- it's like you and the person you want to "communicate" with put two marbles, one red and one black, into two bags. You randomly pick one, your partner takes the other. You fly apart at 0.9c for a while. Then you look in your bag. It's a red marble. You now "instantly" know that your partner has a black marble -- but you haven't actually communicated anything.
* It's just an analogy; the fact that it doesn't obey Bell's theorem is immaterial to understanding why you haven't communicated anything.
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