First pure quantum network is here, promises perfect security

It doesn't get much more futuristic than "universal quantum network," but we're going to have to find something else to pine over, since a UQN now exists. A group from the Max Planck Institute of Quantum Optics has tied the quantum states of two atoms together using photons, creating the first network of qubits.

A quantum network is just like a regular network, the one that you're almost certainly connected to at this very moment. The only difference is that each node in the network is just a single atom (rubidium atoms, as it happens), and those atoms are connected by photons. For the first time ever, scientists have managed to get these individual atoms to read a qubit off of a photon, store that qubit, and then write it out onto another photon and send it off to another atom, creating a fully functional quantum network that has the potential to be expanded to however many atoms we want.

How Quantum Networking Works

You remember the deal with the quantum states of atoms, right? You know, how you can use quantum spin to represent the binary states of zero or one or both or neither all at the same time? Yeah, don't worry, when it comes down to it it's not something that anyone really understands. You just sort of have to accept that that's the way it is, and that quantum bits (qubits) are rather weird.

So, okay, this quantum weirdness comes in handy when you want to create a very specific sort of computer, but what's the point of a quantum network? Well, if you're the paranoid sort, you're probably aware that when you send data from one place to another in a traditional network, those data can be intercepted en route and read by some nefarious person with nothing better to do with their time.

The cool bit about a quantum network is that it offers a way to keep a data transmission perfectly secure. To explain why this is the case, let's first go over how the network functions. Basically, you've got one single atom on one end, and other single atom on the other end, and these two atoms are connected with a length of optical fiber through which single photons can travel. If you get a bunch of very clever people with a bunch of very expensive equipment together in a room with one of those atoms, you can get that atom to emit a photon that travels down the optical fiber containing the quantum signature of the atom that it was emitted from. And when that photon runs smack into the second atom, it imprints it with the quantum information from the first atom, entangling the two.

When two atoms are entangled like this, it means that you can measure the quantum state of one of them, and even though the result of your measurement will be random, you can be 100% certain that the quantum state of the other one will match it. Why and how does this work? Nobody has any idea. Seriously. But it definitely does, because we can do it.

Quantum Lockdown

Now, let's get back to this whole secure network thing. You've got a pair of entangled atoms that you can measure, and you'll get back a random state (a one or a zero) that you know will be the same for both atoms. You can measure them over and over, getting a new random state each time you do, and gradually you and the person measuring the other atom will be able to build up a long string of totally random (but totally identical) ones and zeros. This is your quantum key.

There are three things that make a quantum key so secure. Thing one is that the single photon that transmits the entanglement itself cannot be messed with, since messing with it screws up the quantum signature of the atom that it originally came from. Thing two is that while you're measuring your random ones and zeroes, if anyone tries to peek in and measure your atom at the same time (to figure out your key), you'll be able to tell. And thing three is that you don't have to send the key itself back and forth, since you're relying on entangled atoms that totally ignore conventional rules of space and time.*

Hooray, you've got a super-secure quantum key! To use it, you turn it into what's called a one-time pad, which is a very old fashioned and very simple but theoretically 100% secure way to encrypt something. A one-time pad is just a completely random string of ones and zeros. That's it, and you've got one of those in the form of your quantum key. Using binary arithmetic, you add that perfectly random string of data to the data that make up your decidedly non-random message, ending up with a new batch of data that looks completely random. You can send that message through any non-secure network you like, and nobody will ever be able to break it. Ever.

When your recipient (the dude with the other entangled atom and an identical quantum key) gets your message, all they have to do is do that binary arithmetic backwards, subtracting the quantum key from the encrypted message, and that's it. Message decoded!

The reason this system is so appealing is that theoretically, there are zero weak points in the information chain. Theoretically (and we really do have to stress that "theoretically"), an entangled quantum network offers a way to send information back and forth with 100% confidence that nobody will be able to spy on you. We don't have this capability yet, but with this first operational entangled quantum network, we're getting closer, in that all of the pieces of the puzzle do seem to exist.

Via Scientific American and Wired

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*If you're wondering why we can't use entanglement to transmit information faster than the speed of light, it's because entangled atoms only share their randomness. You can be sure that measuring one of them will result in the same measurement on the other one no matter how far away it is, but we have no control over what that measurement will be.