So if quantum entanglement does not enable FTL communication, what is it good for? How do you keep getting entangled particles from one place to another?
Chinese scientists test quantum entanglement over unprecedented distance
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Re: Chinese scientists test quantum entanglement over unprecedented distance
#62Earlier quoted context omitted.
Not a physicist either, but as I understand it, entanglement is not useful for communication on its own, but allows for tamper-proof key exchange. Basically, you generate a pair of entangled particles. Keep one of the pair and send the other away. Now both you and your partner randomly select a basis to measure the particle in. After a few repetitions, you tell each other (over a classic channel) how you measured and…
Isn't that no better than a one-time pad if you need a classic channel?
Quantum key-exchange solves the distribution problem for one-time pads. And a one-time pad is the best security you could possibly get, so I'm not sure what you mean by "no better", since that's a given.
Re: Chinese scientists test quantum entanglement over unprecedented distance
#63I thought quantum entanglement is the idea that two quantum particles separated by distance will behave the same at the same time regardless od distance. Why is sending photons over distances a verification of quantum entanglement? Can someone elaborate on what this is?
I too am wondering about this. I thought entanglement couldn't be used for communication because observing both particles simultaneously will reveal that they have opposite spins. It's a conformation after observation, based on what the quantum superposition collapses into once you observe the state. But the actual state is also unknown until you observe it. I'm not a physicist, but I'm sure there're be one on HN tha…
I am a physicist.
I only glossed the article, but from what I gathered, this didn't prove anything about QM, rather it was a technical achievement. One of the biggest problems with entangled pair distribution is actually separating the particles over some distance, because when the entangled particles interact with something ("observed") their state collapses and the entanglement may end.
This is what the achievement was, they managed to not destroy entanglement of a pair over a very long distance.
Re: Chinese scientists test quantum entanglement over unprecedented distance
#64Earlier quoted context omitted.
[I'm not a physicist] I like your analogy for color entanglement. However, it doesn't show the "weirdness" of superposition (the balls don't have a concrete value until one of the boxes are opened, at which point, this triggers the determination of the open box's color value), nor the "spookiness" of action-at-a-distance (opening one box instantly influences the state of the ball in the other box -- which now must be…
> the balls don't have a concrete value until one of the boxes are opened, > opening one box instantly influences the state of the ball in the other box How do you know that without opening the box?
Bohr and Heisenberg interpret it that way.
https://en.m.wikipedia.org/wiki/Copenhagen_interpretation
And Schrödinger's cat never told us what really happened inside the box :)
Re: Chinese scientists test quantum entanglement over unprecedented distance
#65Earlier quoted context omitted.
[I'm not a physicist] I like your analogy for color entanglement. However, it doesn't show the "weirdness" of superposition (the balls don't have a concrete value until one of the boxes are opened, at which point, this triggers the determination of the open box's color value), nor the "spookiness" of action-at-a-distance (opening one box instantly influences the state of the ball in the other box -- which now must be…
> the balls don't have a concrete value until one of the boxes are opened, > opening one box instantly influences the state of the ball in the other box How do you know that without opening the box?
This analogy is confusing because it is incorrect in some fundamental ways. You aren't just opening a box and seeing one or the other color. The measurement itself determines a role in the outcome that you couldn't predict in advance.
It is more like you get to choose to measure only one of the red, green or blue channels. If you send a message containing which channel and the measurement, the other person can do the same measurement and find the complementary value. Without the channel and the original measurement the other person just sees random behavior.
The spooky part is that I don't choose the color channel to measure until after the entangled balls are separated
Re: Chinese scientists test quantum entanglement over unprecedented distance
#66A staple of sci-fi has been a pair of boxes with quantum entangled particles inside allowing FTL communications once the boxes travel (sub-FTL) to their destination. By my understanding that's impossible, although I always wondered if you could coordinate FTL, by detecting when a particle was no longer entangled. Can someone with the background comment?
A few weeks ago there was another announcement about the advancement in arresting the evolution of a quantum system
https://news.ycombinator.com/item?id=14347231
In this way it seems to me communication may be possible
A sort of sending bytes stead bits
Re: Chinese scientists test quantum entanglement over unprecedented distance
#67Note that while particles can be entangled, the special/useful thing is the postponement of further entanglement (with the rest of the world).
Useful for what? Every time I see someone propose a use for QE someone else comes along and assures us it won't work.
Re: Chinese scientists test quantum entanglement over unprecedented distance
#68Earlier quoted context omitted.
But in principle could we access the history?
How? You can't just keep measuring the position of the particle and keep a list. Recording quantum information entangles the recorder and the system under test, which affects how interference plays out, which affects the later parts of the path in a way that makes your path-computer not work.
How? Think a bit harder. It's pretty obvious once you get it. John Titor explained it to me last year.
I'm just kidding. Yeah, what you said makes sense. How do we know though that IN PRINCIPLE we can't figure out which QM interprtation is correct? Because we are sure there is no additional data/math that can be developed?
Re: Chinese scientists test quantum entanglement over unprecedented distance
#69Earlier quoted context omitted.
Caveat: I only have a B.Science degree in Engineering Physics (Colorado, USA) and hobbyist-type research in PhD-level physics. I've always found it easier to explain quantum entanglement thusly: Imagine you have 2 cubes that alternate glowing between purple and orange. And they stay in one color for a random amount of time (100s of milliseconds). Holding both in your hands, you would just have randomly color-shifting…
> 1. they will be locked into one color No, this is wrong! As someone else commented, this is just not knowing something. The weirdness of the quantum entanglement comes from the fact that the behavior of each cube is experimentally correctly described by equations based on the cubes continuing to change color randomly.
I probably should have clarified that my analogy was coming from a perspective of super-deterministic[1] compatibilism[2]. In the absence of knowing the truth, I feel that solution is the best model / explanation. It strips away the 'spookiness' and many other problems, though it admittedly has its own issues with testability and proof.
I do not believe in the idea that particles can super-luminally affect each other during the moment of observation. I think that is a failing of our measurement systems and ability to describe what's going on. Just as we often fail to accurately convey the Schrodinger's Cat thought experiment [3].
But for the record, if a different perspective were definitively proven to be true, I would of course switch to it, as any scientist should. But given that there are multiple avenues of investigation open, I'm exploring super-deterministic compatibilism.
[1] https://en.wikipedia.org/wiki/Superdeterminism
Re: Chinese scientists test quantum entanglement over unprecedented distance
#70Earlier quoted context omitted.
With quantum entanglement, what happens to one particle prior to measurement affects the other even if the effect occurred after they were parted. That's where it gets really spooky. To belabour the colours and boxes analogy: We have 2 balls, if one is orange the other is always purple and vice versa. Now these balls have an extra property. If you shake them vigorously they may or may not change colour randomly. So,…
Sounds like they never were two separate entities to begin with or never parted location. Which is spooky too, in a different way.
Experimentally, when they entangle photons, they just pick two random photons.
If two random photons used in an experiment weren't ever really separate entities to being with, even before the experiment ever started, then one should probably conclude that /all/ photons are not really separate entities.
Otherwise, every quantum experiment we've ever done has just /happened/ to pick two photons that were actually one entity, unlike most other photons.