This abstract of an article from Science talks of a possible quantum communication network based on entangled particles and their seeming ability to instantly respond to each other whether separated by a few kilometres or the breadth of the entire universe.
I’ve read something of entanglement before but never understood it at all. How can particles affect each other over interstellar distances? What happened to the theory that nothing including information goes faster than the speed of light? This is confusing stuff!
It is important to make a very clear distinction between FTL transfer of information and FTL interaction.
This study is not claiming FTL transfer of information but is directed at providing ultrasecure communications.
The speed of light/causality is not being violated in this case, and there are several well known effects that can happen FTL like the expansion of the universe itself.
This effort is related to security and not speed, quantum entanglement does not imply faster than light communication. You cannot affect which state the particle goes into, even though it doesn’t ‘decide’ on its state until you observe it.
It is this property that actually makes this interaction useful for encryption.
Communication speed is still limited by that pesky little ‘c’
In particular, quantum security is an anti-tampering measure, which protects against man-in-the-middle attacks.
Imagine, for instance, that you’re trying to log into some website (say, the SDMB). You send your password, and then you see the “Thank you for logging in” message, and you do whatever you wanted to do on the site. But unbeknownst to you, when you sent the password originally, you weren’t actually communicating directly with the site you wanted to log in to, but to some scammer. You tell him your password, and then he records a copy of it, and then he tells the real site the password, and then the real site serves him the “thank you for logging in” page, and then he sends that back to you. It looks legit to you, but meanwhile he’s stolen your password.
If, instead, you used a piece of quantum information for your password, this would be impossible, because quantum information cannot be copied. The attacker could still intercept the password, but he’d have to choose between keeping the copy himself (in which case you’d see that you weren’t logged in, and know something was wrong), or passing it along without making a copy, in which case he hasn’t gained anything.
But like all things cryptographic, the difference between theory and implementation is vast. Two examples of cracks into quantum cryptography are here and here.
There’s been enough of these approaches discovered over the years that it might not be possible in practice to be 100% sure of protected communication. Some new exploit may be found at some point. Some caveats are needed, as usual.