h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Is it true that in Quantum Theory if two particles collide they mimic or mirror each others behaviour? And is it also true they can do this over any distance instantaneously?

If this is the case it would revolutionise computing and communications.

I heard about this phenomena on a radio program and I think it was called entanglement.

I'm no expert on quantum theory so I expect someone will post more correct details but I have read a little on this.

You have your understanding a little awry but your definitely getting there

Quantum entanglement is a property of two particles that arise from the same source (as I understand it). Since they arise from the same source there must be conservation of energy, momentum and such for the process to be obeying the laws of physics.

Consequently if, say they are photons, then if one is polarised to the right by say 90 degrees, then the other is polarised to the left by 90 degrees. Until we observe them though, we do not know which is which.

Now fire these two photons down fibre optic cables going in different directions. It turns out that when you measure one, and say you get the 90 degree right polarisation property, then the other is instantaneously the other (90 deg left). This stands true even if they are now hundreds of miles apart. So in essence one has 'teleported' this information hundreds of miles instantaneously.

This is, and always has been the case. It may or may not revolutionise communication. The tricky bit is finding a practical use for it. I understand they're working on it

That's my understanding as well, although there seems to be some fundamental laws which indicate communication will not be possible - that allows special relativity to hold (nothing is transmitted faster than the speed of light).

<>

i just read this and it strikes me as there is no transfer of information

if you take two marbles a red one and a blue one but dont know which is which and don't look at then and roll them away from you in different directions then look at one and it is red then the otherone must be blue.

that is just common sense

or am i missing some thing

It's very intriguing

Taff - you're correct in principle.

Unfortunately, the current maximum for firing a polarised photon down a fibre optic and guaranteeing that it remains polarised is about a mile. And that can only happen if there aren't lots of other polarised photons chasing it down the cable.

So there's a way to go on that front.

B

<>

i read and understood every word....you still lost me there

In other words, we can send polarised photons a mile if we do them one at a time. Other photons provide electromagnetic disturbance that mucks up whatever we wanted the first one to do.

B

>i just read this and it strikes me as there is no transfer of information<

I'm not sure I'd disagree. Quantum entanglement *is* an interesting piece of physics and I think one of Einstein's famous 'consequence' of quantum theory he predicted rather disparagingly, thinking it would disprove it. Not so, in fact his prediction is all too correct.
However, if you read around all the headlines, I'm not convinced it's much more of a curiosity than anything else.

We'll see...

As I understand it, it is about how you can make an object one thing, depending on how you observe it. Imagine you have two balls, which are sent in opposite directions very fast. You do an experiment to one, which has the effect of giving the ball a blue colour where before it was a blueish-reddish colour. One ball will be blue and one will be red, and since this one is now blue the other one will need to be red. If you look at the other ball you will find this to be true. However there was not enough time for information to move from the one ball to the other, saying "Hey I just turned blue, you need to turn red" if that information moved below the speed of light.

A Quantum computer is in the offing, according to 'Science Daily' it will solve problems in a few hours where the current lot would take thousands of years,

They are inherently imprecise though.

I don't think they are imprecise inherently. True we haven't got enough qbits yet but there is no inherent reason why we can't get enough to compare to normal PCs accuracy.

No they are meant to be imprecise, due to the nature of quantum mechanics. I attended a lecture on it ages ago, and it´s one of the few things I remember.

what do you mean by precise Xanatic? In what way? I mean, they give the 'right' answer do they not? I presume they must otherwise we wouldn't want to build them?
I guess they could give the wrong answer sometimes (nothing is 100% reliable), but then so can a current processor. And so do rna/dna processors.

If two particles are entangled, when you change one of them the other one will also be affected. Something that apparently happens instantaneously.

Unfortunately, you can't continuously watch the other one, so you can't say to yourself "it's just changed, so I've received a message".

By the time you work the maths out, it turns out that you can't receive any useful information any quicker than those slow old light beams could have got it to you anyway. Apparently .

cheers, Martin

Well, I'll stick to pure maths. If you managed to move them very far apart, would they still mirror each other?

"If you managed to move them very far apart, would they still mirror each other?"

There's no theoretical limit on distance, I believe.

According to http://space.newscientist.com/article/dn14113-space-station-to-be-quantum-communications-hub.html?feedId=space_rss20 :-

"The current distance record for transmitting entangled photons stands at 144 kilometres, between telescopes in the Canary Islands and Tenerife."

The article discusses a suggestion to use the Space Station as a hub for quantum-entangled communications:-

""Using a satellite, we can connect between points a few thousand kilometres apart," says Zeilinger. "Ultimately, we could use quantum cryptography between any two points of the world.""

BTW, quantum cryptography relies on photons remaining entagled from one end of the comms link to the other.


This isn't an attempt to send data faster than light. If someone intercepts the "sent away" photons, it will reduce the level of entanglement with their stay-at-home twins, effectively corrupting 50% of the bits (in the simplest schemes). Sender and receiver then use the data transmitted over the quantum link as a key to encrypt comms over normal routes, eg over the internet. If the data is garbled when it is decrypted, then the key has been "snooped", and must be discarded.

cheers, Martin

Maybe I'm missing something, but want if you could produce entangled particle pairs at point C, then fired them off, one to point A and the other to B.

A<=====(C)=====>B

If you affect a particle at point A, presumably in a controlled way, say, left spin or polarization for 1 and right for 0, wouldn't someone at point B be able to decypher the message you are sending in that way?

--_-_ _-<A<=====(C)=====>B>_--_-_ _


Taking A and B to be very far away from each other, it would conceivably produce the type of effect seen in sci-fi movies where ships are communicating instantly with one another.

Or like I said, am I missing something?

As a simplified analogy, imagine a dice which is constantly being thrown and re-thrown. The top face value will be constantly changing.

With quantum entanglement, the dice at the other end of the link is coordinated, such that it is always the other way up. The opposite sides of a dice always add up to seven, so if you read one dice you can work out the value on the other one. This is not a perfect analogy, but gives a flavour.

If someone at the sending end of the link briefly forces their dice to point to "four", the other end will briefly read "three". Trouble is, if you were watching the dice at the other end, how would you know that the "three" that briefly appeared was the value you were supposed to be watching for?

As I understand it, a signal has to be sent via the slow route saying "wait for it... wait for it... get ready... OK, read it... NOW".

The entanglement doesn't carry any *information* from point to point, so you can't use them to directly communicate from point to point at superluminal speeds.


Not sure if I should throw in another analogy, but what the hell...

Think of entanglement as one of the hidden things that goes on inside the machinery of quantum mechanics to keep it working.

Consider the passengers inside a car as the message that is passed by the communication link. Passengers (information) cannot reach their destination any faster than the maximum speed of the car (which is equivalent to the speed of light in this analogy), regardless of how fast electrical signals move around "under the hood".

This means you can never have your passengers arrive at their destination faster than the "speed-of-light" limit (maximum speed of vehicle) would allow, it's just one of those internal things that keeps the machinery going.

cheers, Martin