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View Full Version : Looking for information on Quantum Communication


odaran
01-20-2005, 02:24 PM
I know I don't post much, but I need help finding information. I know that not too long ago(maybe a year or so) there was some sort of breakthrough about paired atomic particles that react in the same way when one is influenced by an outside sorce. If I remember right, there was even discussion on this message board about this. For the life of me, I am not able to find this using the search function. Google, the great tool that it is, comes up with research and information from years ago(1987 for example). I am wondering if anyone knows the threads from this message board and direct me to them, so I can go through them and find links. Or maybe even links to other websites with the newer information on them.

I thank you all and always, you guys rock and I have learned so much. :)

GorillaMan
01-20-2005, 02:33 PM
Searching www.newscientist.com for quantum communication turned up plenty of results, such as this (http://www.newscientist.com/article.ns?id=dn43).

Revtim
01-20-2005, 02:50 PM
Searching on "quantum entanglement" might turn up some results too.

Whack-a-Mole
01-20-2005, 02:50 PM
If the following is too much of a hijack let me know and I'll get it moved to another thread. It does not directly answer the OP (which I think GorillaMan already did) but bears on what the OP is about.

I still do not get why quantum entanglement cannot be used to transmit information. Consider the following experiment (based on the article GorillaMan linked to):

Assume I have gone to Alpha Centauri and am interested in whether the Chicago Cubs won the world series (this is a hypothetical so no need to point out the chances of that happening exceed the expected life of the universe).

I arrange with a colleague who is staying on earth to take one pair of a quantum entangled pair and set it to spinning round and round in a circle here on earth while I take the other particle to Alpha Centauri. My colleague knows I will arrive, say, 30 years from today. I tell him that in 30 years and one day stop your particle spinning around if the Cubs won the series in that time...otherwise leave it be. At my appointed time I observe my particle. If it has changed its spin I know the Cubs won. If not I know they still haven't managed it.

Isn't that then sending information faster than the speed of light?

micco
01-20-2005, 03:26 PM
This recent article (http://www.sciam.com/article.cfm?chanID=sa006&articleID=000479CD-F58C-11BE-AD0683414B7F0000&ref=rdf) has some info. Particularly the part about experiments with entangled pairs discusses using these effects for communication.

Note that the "quantum encryption" discussed is really just a method of encryption key distribution. It is a communication device.

Omphaloskeptic
01-20-2005, 04:11 PM
I still do not get why quantum entanglement cannot be used to transmit information. Consider the following experiment (based on the article GorillaMan linked to):

Assume I have gone to Alpha Centauri and am interested in whether the Chicago Cubs won the world series (this is a hypothetical so no need to point out the chances of that happening exceed the expected life of the universe).

I arrange with a colleague who is staying on earth to take one pair of a quantum entangled pair and set it to spinning round and round in a circle here on earth while I take the other particle to Alpha Centauri. My colleague knows I will arrive, say, 30 years from today. I tell him that in 30 years and one day stop your particle spinning around if the Cubs won the series in that time...otherwise leave it be. At my appointed time I observe my particle. If it has changed its spin I know the Cubs won. If not I know they still haven't managed it.

Isn't that then sending information faster than the speed of light?First, a commentary on the New Scientist article linked by GorillaMan above: It's somewhat misleading, in that it uses the terms "information" and "communication" in a nontechnical sense. This conveys some of the strangeness of entanglement but does NOT mean that actual "information," in a technical sense, is being transmitted. Although you don't refer directly to this article, your idea uses a setup which is formally equivalent to their setup; this distinction is important in understanding why your idea doesn't work.

Basically, the problem is that nothing your friend can do to his electron can instantaneously change anything you can measure about your electron. Your friend can learn something about the state of your electron, but he can't change it. This is easy to demonstrate mathematically, once you know the formalism of quantum mechanics.

A reasonable classical analogy is to consider pulling a playing card apart (front-to-back) and putting the front half (with the pips) in one envelope and the back half in another. Then, after mixing the envelopes up, your friend takes one and you take the other. Later, by opening only his envelope, your friend (instantaneously!) knows which half of the card you have. But there's no "communication" going on here. He can't mark his half of the card and have the markings somehow show up on your card. It's just a probabilistic correlation between the two random variables.

Mathochist
01-20-2005, 04:20 PM
A reasonable classical analogy is to consider pulling a playing card apart (front-to-back) and putting the front half (with the pips) in one envelope and the back half in another. Then, after mixing the envelopes up, your friend takes one and you take the other. Later, by opening only his envelope, your friend (instantaneously!) knows which half of the card you have. But there's no "communication" going on here. He can't mark his half of the card and have the markings somehow show up on your card. It's just a probabilistic correlation between the two random variables.

The standard example I usually heard (and now give) is of a "magic" pair of decks of cards which will always turn up the same card, but only if they're both perfectly randomly shuffled. If you stack your deck to encode a message, the link is broken and your friend's deck won't turn up the same cards.

Omphaloskeptic
01-20-2005, 04:40 PM
The standard example I usually heard (and now give) is of a "magic" pair of decks of cards which will always turn up the same card, but only if they're both perfectly randomly shuffled. If you stack your deck to encode a message, the link is broken and your friend's deck won't turn up the same cards.That's a closer analogy to the real quantum situation, but it makes use of a nonintuitive "magical" feature. I like the above analogy because it shows similar behavior in an entirely classical system.

Mathochist
01-20-2005, 04:47 PM
That's a closer analogy to the real quantum situation, but it makes use of a nonintuitive "magical" feature. I like the above analogy because it shows similar behavior in an entirely classical system.

As far as I can tell, most people outside math and physics regard math and physics in general (including Newtonian mechanics and basic algebra) as nonintuitive "magic". I don't see a great loss. </rant>

Still, you've got a point.

Enola Straight
01-21-2005, 12:43 AM
Perhaps quantum entangles particle pairs are actually "tangent" in some hyper-dimentional fashion we don't directly sense: the "actual" distance (and, due to lightspeed's finite velocity, time interval) is irrelevant.

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