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Chinese scientists test quantum entanglement over unprecedented distance

scientificamerican.com

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Re: Chinese scientists test quantum entanglement over unprecedented distance

#51

Post by lucozade »

novalis78 wrote: post #40
lucozade wrote: post #37

Earlier 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.

When I first started to be interested in this area, it occurred to me that it might be something like a 'stretchy' spacetime i.e. the particles are actually locally connected but appear unconnected if you use a static spacetime as a reference as we tend to. I guess something analogous to the idea that someone moving close to c experiences time different from someone in a slow moving reference frame.

Unfortunately, I have neither the experience nor ability to progress that thought much further so, if someone wants to go off and get a Nobel from it, feel free...

Re: Chinese scientists test quantum entanglement over unprecedented distance

#52

Post by pishpash »

indolering wrote: post #30
sliken wrote: post #6

A 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?

Damn, all of these explanations are overly complex! The most basic reason it doesn't work is because measuring your particle breaks the entanglement. So you would know that your partner has the opposite spin (or whatever) but you can't bootstrap any sort of communication system with that.

Not overly complex, but overly simple. I wish people talking about physics learn for the last time that analogies are a horrible way to explain a complex concept that at best adds an unnecessary translation layer increasing cognitive load and at worst hand waves away the most critical parts.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#53

Post by RugnirViking »

yorwba wrote: post #29
djsumdog wrote: post #7

Earlier quoted context omitted.

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…

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?

Re: Chinese scientists test quantum entanglement over unprecedented distance

#54

Post by EGreg »

Strilanc wrote: post #49
EGreg wrote: post #8

Wikipedia says: " If Bohmian Mechanics as an non-local hidden variables interpretation of quantum mechanics is accurate it should allow quantum computers to implement a search of an N-item database at most in O ( N 3 ) {\displaystyle O({\sqrt[{3}]{N}})} steps. This is slightly faster than the O ( N ) O({\sqrt {N}}) steps taken by Grover's algorithm using standard interpretations of quantum mechanics. Neither search m…

It's just a wrong statement. Follow the reference to the paper and see: > [...] we show that if we could examine the entire history of a hidden variable, then we could efficiently solve [...] Bohmian mechanics doesn't give you access to the entire history of a hidden variable.

But in principle could we access the history?

Re: Chinese scientists test quantum entanglement over unprecedented distance

#55

Post by wiradikusuma »

swashbuck1r wrote: post #47
heimdall wrote: post #17

Earlier 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…

[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?

Re: Chinese scientists test quantum entanglement over unprecedented distance

#56

Post by felipemnoa »

novalis78 wrote: post #40
lucozade wrote: post #37

Earlier 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.

>>Sounds like they never were two separate entities to begin with or never parted location. Which is spooky too, in a different way.

Sounds interesting. I wouldn't be surprised if it turns out that these two particles are really just a single particle in a different dimension that we don't know it exists yet.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#57

Post by dleslie »

MrZongle2 wrote: post #50
greeneggs wrote: post #46

Earlier quoted context omitted.

> A 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. Of course this is complete nonsense. It's useful for plotting stories, but seems to have imprinted on a large fraction of readers a total falsehood. Do you know where the idea started? I think the first time I saw it was in Orson Scott Card's novel "En…

Looks like it may have started in 1966 with a story by Ursula K. Le Guin. She called the device an ansible , which is what it was also called in OSC's books. https://en.wikipedia.org/wiki/Ansible

From the book The Dispossessed.

It's a great novel, by the way.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#58

Post by kbenson »

im3w1l wrote: post #24
gabrielgoh wrote: post #19

Earlier quoted context omitted.

i fail to see whats mysterious about knowing something without direct proof. You could put a literal orange cube in a box, ship it across the galaxy, and you'd "know" anyone opening the box would be an orange cube. You don't need to be present at the unboxing to know that with certainty. How is this different? Am I missing something?

Allow me to give a more (but not perfectly) accurate analogy. I give you a magic piece of paper with a two binary digits (00/01/10/11) on it. I also keep such a paper for myself. If we both start by looking at our first digits then those will mysteriosly match, but if we then look at our second digits they will have just a 50% chance of matching. However, if we both start by looking at the last digits, then those wil…

It sounds like an interesting, if very expensive, way to get around primes for key exchange, in the world where prime factorization is much more efficient. Need a random number, look a bun of shared digits in a predefined order. You now have a shared random number that's perfectly secure, as long as you shipped out your shared state in advance.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#59

Post by Strilanc »

EGreg wrote: post #54
Strilanc wrote: post #49

Earlier quoted context omitted.

It's just a wrong statement. Follow the reference to the paper and see: > [...] we show that if we could examine the entire history of a hidden variable, then we could efficiently solve [...] Bohmian mechanics doesn't give you access to the entire history of a hidden variable.

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.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#60

Post by pif »

heimdall wrote: post #17
sliken wrote: post #6

A 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?

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.

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