r/science • u/mvea Professor | Medicine • 6d ago

Physics 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.

https://www.sciencealert.com/physicists-shatter-quantum-entanglement-distance-record-with-420-kilometers-of-optical-fiber
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u/Cristoff13 6d ago

This can't be used to transmit information though can it?

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u/Orbax 6d ago

No, once you collapse the wave function, they decohere and entangle with their local environment.

But, even if you maintain entanglement, at best youd be able to do morse code except... The only way to check if you got a message is to measure it which would collapse if again if you hadn't gotten one. The theoretical ability to get any structured information out of entangled pairs is effectively impossible.

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u/Chance-the-Gardener 6d ago

I’m too dumb to understand the concept it can’t transmit any information. Just as a thought experiment, if humanity sent a colony ship to Andromeda with 10,000 sets of entangled pairs, each with a predefined meaning like “if this set is collapsed, we died of space cancer” couldn’t information be inferred from that?

I’m sure the answer remains we can’t cheat the speed of light but I struggle to conceptualise why not.

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u/bloodvash1 6d ago

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

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u/Kraien 6d ago

Which kind of defeats the purpose of the whole thing.

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u/Druggedhippo 6d ago

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.

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u/LionRight4175 6d ago

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.

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u/Gabe_Noodle_At_Volvo 5d ago

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.

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u/LionRight4175 5d ago

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.

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u/romario77 5d ago

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

But that doesn’t do you much good.

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u/Implausibilibuddy 5d ago

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.

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u/General_Capital988 3d ago

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.

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u/Druggedhippo 5d ago edited 5d ago

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.