this post was submitted on 28 Mar 2025
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I thought the whole point of entangled communication is that you didn’t need to “send” anything. It automatically flips the entangled bit on the other end, all that “spooky action at a distance” bidness. Why do the need to “send” entangled photons?
Quantum physicist here. Your idea is effectively correct, but the issue lies in generating entanglement at a distance, which is a gargantuan task. You can't start with two qubits (in the current discussion the photons are qubits, holders of quantum information) and simply proclaim them to be entangled over long distances (even centimeters can be considered long in the quantum realm). One of the more promising methods to achieve entanglement at a distance is to create entangled photons locally in your friendly neighborhood lab, and send them on their merry way. Photons are incredibly good at travelling far. When they have reached their destination you are free to do the next complicated part, the 'spooky action at a distance' as you call it ;) I just call it magic.
The two standardized method of sending photons over earthly distances is either a) via air (e.g., lasers, radiowaves or satellite communication) or b) via fibre optics. Since the fibre optics network is so developed across the globe, quantum information engineers would love to tap into that infrastructure - which is the main motivation for the work done in the main article. Here, they proved that the entanglement survives the journey through the optical cable, which was expected (but not a given) for short distances. Entanglement is sensitive business and is lost very easily. 30 km of travel through an optical cable can be considered very, very long based on these premises - but also around the upper limit of what can be achieved without significant advances of quantum repeaters which replaces the functionality of amplifiers in traditional optical fibre networks.
I've heard this explanation of it once from a physicist: Imagine you have a photograph. You rip that photograph in half. Now you put both halves into envelopes and mix them up. At this point, you don't know which half is in which envelope. Now you send one of the envelopes to Australia. You open your half. Because you see that you have the left half of the photograph, you gain instant knowledge that the right half is in Australia.
With quanta, you can for example have a subatomic particle which decays into two quanta and then you know those quanta to have certain similar properties. As Wikipedia puts it:
are the ripped photographs duplicates since changes on one effects the other im confused, like does ripping create two of the same
Right, so you might want ask about this on !askscience@lemmy.world or such, as science-y folks tend to not be comfortable with what I'm about to say, but to the best of my knowledge, that's all just complete horseshit. Like, no, your understanding of the photograph is not somehow incorrect. It's just two halves of a photograph and because you know the first half, you know what's on the second half. The second half does not get changed by you looking at the first half. Nor does the entangled quantum get changed by you looking at the first quantum.
I think, a big part of this mass confusion is that at the size that quanta have, looking at them does actually change/move the quantum that you look at (not a potentially entangled one). This is not for crazy reasons, but because looking at them requires light, which is the equivalent of blasting them with photons, and photons are themselves quanta.
It's like if you had a dark room with a ball in it and you can only throw other balls into there to try to figure out where the first ball is. You need to hit the first ball, in order to have a chance of working out where it might be based on the angle that your thrown ball returns at. If you do hit the first ball, it will move. So, you only really know where it was at the time of impact. Quanta are not balls, but they do still interfere with each other when they get close to each other.
Entanglement in this analogy is that you've spun up two balls next to each other like cogwheels, so you know them to have the opposite (and equally strong) spin. Then you've released those into the dark room and start throwing other balls at them to try to work out their spin. If you hit one of the spinning balls, your thrown ball will come back out with a spin opposite to that and the spin of the ball that was hit will have reduced. In this moment, you know that the other spinning ball also has an opposite spin, because you originally spun the two balls like cogwheels. The other ball does not get changed by you measuring the first, but there's no way for you to know, because you have to measure it to find out, which means also throwing a ball at it and therefore changing it, too.
As far as I can tell, this is the other big part of where the confusion comes from. Because measuring necessarily also involves changing the thing and because it's actually impossible to disprove that the entangled quantum didn't get changed by us measuring the first, you get folks that follow a school of thought of things being non-deterministic. Of things only being set in stone once you measure them. There's lots of vested interest in things being non-deterministic for religious or moral reasons and there is no way to disprove it at the quantum level. These folks then propagate concepts like superposition and that when you open the box, you're the one that forces the cat to be killed. (Schrödinger was not one of them, by the way. The cat analogy was a critique of superposition as an idea.)
To my knowledge, there's no evidence for non-determinism (folks will sometimes argue with quantum fluctuation showing it, but it doesn't happen in complete isolation, so that disqualifies it in my opinion) and given that the rest of our reality seems to be perfectly deterministic, I think we should assume the quantum stuff to be like that, too, unless proven otherwise, but unfortunately not everyone goes along with that.
The "spooky action" is really just the determination of a particle's spin on one side meaning you already know the other particle will have an opposite spin. This probably violates locality because you gain knowledge about something that's non-local from a quantum perspective, even though entangled particles have to start local (there are opposing interpretations, like the de Broglie-Bohm). While in fiction this might suggest that changing the state of one particle simultaneously changes the other, in real life this just means extra information you mathematically shouldn't have, and doesn't really lead to FTL information transmission. What it does mean is that if you want secure communications you can use entangled particles to generate a secret key, determining the spin of them on either side, and you can be sure that they haven't been tampered with and that the other side of the communication will be equal and opposite. It's essentially a one-time pad.
You need to prep by sending the entangled particles (photons in this case). The spooky action is when you act on 1, you also act on the other. The useful bit is the uniqueness of the link. It cannot be intercepted without it being obvious and detectable.
Think of it like voodoo dolls. It works at a distance, but you need to make the voodoo doll using a bit of the target, then send/take it elsewhere to stab.
I'm pretty ignorant of physics, but isn't it only certain kinds of ways of acting on the first particle that "affect" the other, namely actions that measure a property of one particle that is correlated with the same property's value on the other? At first you don't know the value of either but you know they're correlated; but then when you measure and collapse the wave function on one and discovered a value for the property, you have automatically collapsed the wave function on the other too, yielding a predictably correlated value. If it were just any kind of action that affects the other particle, you'd be able to use it to sent information instantaneously, which you can't do. So it's not quite like how people imagine voodoo dolls: do something to the doll (make a change to it) and the person feels the effect. But perhaps someone who studies this stuff can help clarify.
That's fairly close. The only proviso is there are some ways to affect the results. You can't send actual information along the link, but you can prove they were in communication. That proof requires information from the sending end however. It's only provable once that information is sent. Basically they communicate faster than light, but can't send information faster than light. Entanglement is weird.
Thank you. That is helpful and clear.
but voodoo dolls dont really work, do they?
Quantum voodoo dolls do. It's annoyed more than a few scientists over the years.
could you give some source to that? I cant find anything relating to quantum voodoo dolls
It was a bit of a joke. Entangled particles act a little like voodoo dolls, with "spooky action at a distance".
ah, world is going so crazy you cant really know what is what anymore
Once you hit quantum mechanics, you need to throw out a lot of your instinctive knowledge, and just follow the maths. How this maps back to our perception is patchy at best. Once you add science reporters, who don't actually understand the core subject, and you get some... interesting results.
In hindsight, "quantum voodoo dolls" is a term I could easily see being used. There are a lot of poorly thought out Wats to try and describe quantum weirdness.