The First Universal Quantum Network 156
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."
"carries the quantum state"? (Score:4, Informative)
So information might be passed around, but it's never actually being shared.
Which isn't much of a network.
Disclaimer - I'm rusty.
Re:Call me when we have instant transfer of data (Score:5, Informative)
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.
Re:Quantum Internet (Score:5, Informative)
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.
Re:Call me when we have instant transfer of data (Score:4, Informative)
Yes, but entanglement cannot violate causality, which is basically what would happen if you transmit information faster than the speed of light. That means that entanglement itself *could* be faster than light, but it has to have some property that mangles any information you try to piggy back on the process so that it is useless as a communication source at FTL speeds.
The problem isn't getting something faster than light, it's being able to make any use of the process to transmit information.
Re:Call me when we have instant transfer of data (Score:4, Informative)
You forget that quantum shit be weird. If you think of particles as particles and their state as a 0/1 variable, then that's totally true. But particles do crazy things. One of the things they do is act like waves if nobody is looking. Entangled particles have to behave like the same sort of thing. In particular, if one of them enters a two slit setup and self interferes, the entangled pair has to also act like a wave and self interfere. This apparently occurs regardless of distance. What this means is that if Alice and Bob have a shared set of atoms. If Alice shoots an atom at a pair of slits, then Bob's atom will self interfere even if shot at an unshielded detector. Now that's not useful for sending messages, because statistically Bob can't tell if it hit where it hit because it's a particle, or because it's a wave. And the quantum equations say the same thing, that statistically the two states cannot be distinguished from random noise. However, the equations do not apply to larger systems, and we don't currently have ones that do.
Now, people assume there must be some quantum effect to prevent this from being used, because superluminal signals are mutually exclusive with causality, and most people assume causality holds. But there's no strong evidence either way at the moment.