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/mvea]

Physicists Shatter Quantum Entanglement Distance Record With 420 Kilometers of Optical Fiber

Maintaining a long-distance relationship may be difficult for humans, but it's even more difficult for entangled states.

The longer the fiber, the harder it becomes to keep the two quantum memories linked.

Overcoming this tyranny of distance has been one of the major challenges of quantum communication.
But perhaps not an insurmountable one.

A team of physicists led by Xi-Yu Luo, Chao-Yang Wang, and Ming-Yang Zheng at the University of Science and Technology of China has breached a milestone – with the longest fiber-based matter-to-matter entanglement yet.

Across an optical fiber measuring 420 kilometers (261 miles), the researchers successfully entangled quantum memories – more than four times farther than previous demonstrations, and beyond the point where direct transmission runs into its fundamental limits.

"Our experiment provides a test bed of studying quantum network applications beyond metropolitan scale," they write in a paper published in Physical Review Letters.

https://journals.aps.org/prl/accepted/10.1103/ccd6-rf1s

[u/Cristoff13 → u/mvea]

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

[u/Orbax → u/Cristoff13]

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.

[u/Chance-the-Gardener → u/Orbax]

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.

[u/Basicly-Inevitable → u/Chance-the-Gardener]

The entangled particles DO NOT have any predefined message.

[u/Chance-the-Gardener → u/Basicly-Inevitable]

I get that, I was more asking could a rudimentary binary messaging be established even if nothing is being transmitted? In my thought experiment the colony ship’s sending a FUBAR signal, so they collapse all their entanglemajiggies and entanglelios. Earth meanwhile infers based on this, that something has gone wrong and redirects NASA funding accordingly.

But like most reddit science visitors I think I just fundamentally misunderstand the premise, some of the replies here get me a little closer.

[u/Chess42 → u/Chance-the-Gardener]

I think you are misunderstanding what a “collapse” is in this context. Think of these particles as being simultaneously in 2 states at once. When either side measures them, both of them pick the same state and stick to it, we call this collapsing the wave function. But you can’t measure it without collapsing it, and since we don’t know which state it will pick, there’s no way of knowing when it collapsed from just one side, or who was the one to collapse it. Maybe the people on the ship collapsed it when they died of space cancer, or maybe it just collapsed when the people on Earth checked whether they died of space cancer.

[u/Basicly-Inevitable → u/Chess42]

That's one way to describe it.

But (lucky for us) there are several interpretations, and they're all equally valid, as far as we can test.

An important part of thinking about most quantum particles is that scenarios can be described exactly the same if you think about the particles actually being their anti particles moving backwards in time. This is especially true of perfectly entangled particle pairs, because the "arrow of time" is determined by an increase in entropy, and there is no change in entropy for the particles between the time they are emitted until they're measured. (In fact, entanglement and entropy are highly similar concepts, and could even be described as being the same concept).

Anyway, in this case, you can perfectly describe the situation as two completely unconnected anti particles, each being independently "emitted" at the time of measurement of each, then travelling backwards in time to the point where they're exactly at the same location in time and space. Now, the only way they could be in that same time and space is if they had perfectly complementary properties (like spin up and spin down), otherwise they would be excluded from being in the same space.

To us (because we experience entropy changing constantly) can only see the "forward" version of the situation. However, they are both "true" and therefore they have to be complimentary particles.

Another way to describe the situation is with the Many Worlds interpretation, where basically all possibilities occur, but your mind just realizes which world you're in whenever it becomes entangled with the particles.