Bounding the speed of ‘spooky action at a distance’ • Hacker News Board

Re: Bounding the speed of ‘spooky action at a distance’

#51

Post by eoinmurray92 » Sun, Apr 07, 2013, 6:56 PM UTC

XorNot wrote: post #45

eoinmurray92 wrote: post #26

Earlier quoted context omitted.

It can be used to share a cipher (one-time-pad) because you and I would know the arrangement of bits that the other person has, and no-one else would/could know. However the measurements on the qubit's are probabilistic, so we cannot control the arrangement of bit that we have. We will only know if the are correlated/anti-correlated.

I still live in hope we'll find a way to use it for communication. A system where you had to use a slower-then-light synchronization signal to maintain the entangled state, but didn't actually transmit information over non-entangled bands, would fundamentally revolutionize communication none-the-less. Being able to send data across the Pacific at essentially the speed of light, which was uninterceptable, would still…

I am quite confident that quantum dots will be able to do this sort of thing soon (10yrs). They are shown to be reliable and resistant to decoherence.

I don't know much about that weak measurement stuff, only in really theoretical situations thus far. But it seems a nice way to probe entanglement.

Toshiba @ Cambridge have recently shown QKD over broadband networks over 90km using single photon sources.

Before that the distance record was held by Tyndall in Ireland.

Re: Bounding the speed of ‘spooky action at a distance’

#52

Post by mircopop » Sun, Apr 07, 2013, 7:28 PM UTC

_of wrote: post #17

Watch out for arxiv.org, these papers may or may not have passed peer review. A lot of junk is deposited there.

"There is no such thing as consensus science. If it's consensus, it isn't science. If it's science, it isn't consensus. Period.” Michael Crichton

Re: Bounding the speed of ‘spooky action at a distance’

#53

Post by icambron » Sun, Apr 07, 2013, 10:10 PM UTC

eoinmurray92 wrote: post #50

icambron wrote: post #47

Earlier quoted context omitted.

Can you explain the one-time pad application a bit more? It seems to me that if Alice and Bob can share a bunch of entangled qbits, they could just as easily share a classical one-time pad. What am I missing?

Im not explaining it very well and its difficult to condense, perhaps I'll write a blog post that goes into more detail for a layman reader.

Let me know if you do; I'm certainly interested.

Re: Bounding the speed of ‘spooky action at a distance’

#54

Post by n3rdy » Sun, Apr 07, 2013, 11:28 PM UTC

gambiting wrote: post #9

n3rdy wrote: post #7

Earlier quoted context omitted.

> Then he didn't understand what you were asking. It can easily be faster than the speed of light because no information can be transmitted through this phenomenon. If its observable then how could information not be transmitted? I'm the furthest thing from a physicist but if quantum entanglement causes an atoms polarity to change because millions of miles away another atom it is entangled with did the same, that wou…

Because just observing the polarity changes it, so you can never be sure what position it was in originally. So you can affect an object that is light years away from you instantly, but it's useless for transmitting information, because while you know which state it is in, the party on the other end does not,and has no way of finding out without affecting the information and making it useless.

> Because just observing the polarity changes it, so you can never be sure what position it was in originally.

That does put a crimp in my universal router plans, but that's really interesting by itself that just looking at the atom affects its polarity.

Re: Bounding the speed of ‘spooky action at a distance’

#55

Post by gambiting » Mon, Apr 08, 2013, 1:38 PM UTC

n3rdy wrote: post #54

gambiting wrote: post #9

Earlier quoted context omitted.

Because just observing the polarity changes it, so you can never be sure what position it was in originally. So you can affect an object that is light years away from you instantly, but it's useless for transmitting information, because while you know which state it is in, the party on the other end does not,and has no way of finding out without affecting the information and making it useless.

> Because just observing the polarity changes it, so you can never be sure what position it was in originally. That does put a crimp in my universal router plans, but that's really interesting by itself that just looking at the atom affects its polarity.

Well it does not _really_ change the polarity.

Think about it this way:

You have two cards(A and B) - you and your friend pick one randomly. Until you look at yours, it's both A and B at the same time. It's the same with your friend's card. So you both own cards that are in a "mixed" A/B state. And then, when you and your friend look at them, the state is "decided".

Therefore, you could send your friend and his card on a rocket million years away from Earth, and then look at the card. Then,you can tell with 100% guarantee which card your friend has. Is that information useful? No,not really. You were able to instantaneously find out the state of an object millions of light years away,but you can't use it to communicate.