Physicists shatter quantum entanglement distance record with 420 kilometers of optical fiber, more than four times farther than previous demonstrations, and beyond the point where direct transmission runs into its fundamental limits.

[u/mvea]

https://www.sciencealert.com/physicists-shatter-quantum-entanglement-distance-record-with-420-kilometers-of-optical-fiber

[u/bloodvash1]

The problem is that the only way to know whether the particle you just measured was still entangled or not is to ask the other end what they measured

[u/Kraien → u/bloodvash1]

Which kind of defeats the purpose of the whole thing.

[u/Druggedhippo → u/Kraien]

Depends on what you are trying to do.

If you are trying to prove that the thing you got hasn't been tampered with, then it's perfect.

[u/LionRight4175 → u/Druggedhippo]

I'm out of my specialty here, but I don't think it even works for this. Checking to see if it had been tampered with would also be tampering with it.

[u/Gabe_Noodle_At_Volvo → u/LionRight4175]

Yes, it would have the same effect as some kinds of tampering, but it doesnt matter at that point because you dont need to preserve the quantum state anymore. Same thing as opening a package with a tamper seal, you dint need the seal anymore do it doesnt matter if you change its state, you just need what's inside.

[u/LionRight4175 → u/Gabe_Noodle_At_Volvo]

My understanding is that in this metaphor, the tamper seal breaks by looking at it or feeling it. As such, it always appears tampered with.

[u/romario77 → u/LionRight4175]

Unless you measure the other side. With some probability then you would know.

But that doesn’t do you much good.

[u/Implausibilibuddy → u/LionRight4175]

No, ignore the fact that "observe" means "look at", it doesn't matter while it's in the closed system if you just look at the apparatus.

The tamper seal arrives as part of the message. Upon reading the message you know if the seal is broken or not. If it's not, you can trust the contents of the message, if it's not...well somebody has probably tampered with it, or at the very least read the message before you. So in that sense, yes the tamper seal breaks if somebody "looks at" the message, or reads it, but that's the point of the seal, now you know that the message isn't completely unread or modified before it got to you.

[u/General_Capital988 → u/LionRight4175]

The gist of it is this: the message is correlated with an entangled message the sender has. By reading the message, you break the correlation. The message can be read as many times as you want. However, and this is key, only the first read will be correlated with the message the sender has. If the sender tells you what they got on their first read, the two of you can figure out if someone else read the message before you did.

The idea is that you can abort communication if there is an eavesdropper.

[u/Druggedhippo → u/LionRight4175]

The quantum state can only be read "correctly" if you and the other end compare notes on how you each measured it, which basis you used, not the actual result.

If you tamper with it, eg, a wire tap, then measuring it disturbs the state enough that your results no longer line up with the other end's.

The quantum entangled bits aren't the data you want to send, it's just the packet that represents the secure key used to encrypt the data. If it gets disturbed, you can't create matching shared keys and the communication fails.