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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

#21

Post by coldtea »

gabrielgoh wrote: post #19
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 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?

That this example has very little to do with entanglement, which is not merely "knowing without direct proof". We do that everyday.

The same way, the issue with Shroedigger's cat is not that "the cat is in a single state, but we just don't know whether that's dead or alive unless we open the box" (as many laymen try to explain it).

That's just "I don't know yet" and has nothing to do with the issue Shroedigger tried to highlight or quantum mechanics (not to mention that this we encounter everyday and we can replicate perfectly with say, a box and a person that throws a dice and based on that releases or not a gas in the box with a cat -- that's not what the paradox is about).

For entanglement, see this:

https://www.sciencealert.com/watch-this-is-how-quantum-entan...

Re: Chinese scientists test quantum entanglement over unprecedented distance

#22

Post by IanCal »

gabrielgoh wrote: post #19
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 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?

If I understand right, this starts to touch on Bell's Theorem.

The description as is means that the situation is identical if there's "spooky action at a distance" or if there's none and you just don't know what's in each box until you open one (and therefore can guarantee what's in the other). There are predictions from this however that when tested just don't work. There is something else going on. I would go further but I'm sure my explanations would contain huge inaccuracies that probably would cause more problems with understanding what's happening so will link you to a couple of things:

https://en.wikipedia.org/wiki/Bell%27s_theorem

http://www.askamathematician.com/2010/06/q-how-it-is-that-be...

I'm sure there are others here who are well versed in this who can chip in.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#23

Post by posterboy »

MichaelBurge wrote: post #10
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?

It's about as likely as reading a byte from an initialized /dev/urandom and measuring its entropy to see if someone removed a hardware RNG.

I'd guess it's impossible without measuring both particles and communication to compare the results.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#24

Post by im3w1l »

gabrielgoh wrote: post #19
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 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 will match, and our first digits will have a 50% chance of matching. If you and I start by looking at different digits, then the digits we read are just standard random bits.

The cool thing about this is that our magic paper will also work instantly even if we are lightyears apart. However, if you decide to look at the last digit and you see a 1, then that tells you nothing about me or which digits (if any) I have looked at, so I can't send you a message with this paper.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#25

Post by rtpg »

panic wrote: post #20
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…

The "spookiness" is more subtle than that. You could argue that the cubes have decided on their color as soon as you close the box. Why doesn't this argument work for real quantum systems? In reality, you're not measuring colors of boxes, but spins of particles. You generate two particles with opposite spins along a known axis. If you measure the spins along that same axis, you always get the same result: one is spin…

But Bell's equality means that either the system is non-local or non-deterministic.

So either it's determined when you close the box, but it's spooky action at a distance. Or it's not determined on box closure (no spooky action) but it has "random-ness".

Re: Chinese scientists test quantum entanglement over unprecedented distance

#26

Post by yorwba »

pulse7 wrote: post #4

The interesting part of "quantum communications" is the impact on security and surveillance. Does "quantum communications" even needs switches and routers? Backdoors don't seem to be possible...

Quantum communication works best if you have a direct connection, because the entanglement is easily destroyed if the particles interact with anything. This is the whole point, you can't intercept it without completely degrading the connection. If you try to redirect your particles through a network of switches and routers, be prepared for lots of dropped packets.

Re: Chinese scientists test quantum entanglement over unprecedented distance

#29

Post by yorwba »

djsumdog wrote: post #7
Illniyar wrote: post #2

I 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…

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 discard measurements made in different bases. The remaining results will be strongly correlated, depending on the quality of the entanglement. By comparing a small part, you can find out exactly how much.

If the correlation is lower than expected, you might have some dust in your measurement apparatus, or an eavesdropper has intercepted a few quanta. If the correlation is good enough, you now both have a value that is secret to anyone who did not measure the particles.

The secret value can then be used as a key to encrypt your communication over the classic channel.

If this description is not satisfying, maybe the Wikipedia article explains it better: https://en.wikipedia.org/wiki/Quantum_key_distribution

Re: Chinese scientists test quantum entanglement over unprecedented distance

#30

Post by indolering »

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.
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