Quantum physics at a distance

http://medienportal.univie.ac.at/presse/aktuelle-pressemeldungen/detailansicht/artikel/quantum-physics-at-a-distance/

by icey • 14 years ago

35 16 14 years ago

medienportal.univie.ac.at

walrus2• 14 years ago

Article ($32): http://www.nature.com/nature/journal/vaop/ncurrent/full/natu...

Preprint: http://arxiv.org/abs/1205.3909

sillysaurus• 14 years ago

Why is it $32? I have about $1k left in the bank. I'm deeply fascinated by quantum mechanics (I spend most days researching optical quantum mechanical phenomena), but I can't afford this.

walrus2• 14 years ago

I don't know. A lot of people have asked that question about scientific publishing.

In any case, just read the preprint instead. It's basically the same.

alecdibble• 14 years ago

Because publishers want your money. This is actually a HUGE problem in academia that has yet to be disrupted.

    "we believe the publisher adds relatively little value to the publishing process... We are simply observing that if the process really were as complex, costly and value-added as the publishers protest that it is, 40% margins wouldn't be available."[0]

[0]: http://en.wikipedia.org/wiki/Academic_publishing

lmirosevic• 14 years ago

Nice to know there's some exciting stuff happening in my city for once.

Roboprog• 14 years ago

Weird. So they can send an FTL signal with quantum coupling/entanglement (or whatever). I wonder how "simultaneity" is set by the universe: average light speed between 2 points to determine average time frame???

If they can make it scale, it would be interesting to see how this affects space exploration vehicles: "get me a subspace channel to starfleet!", er, "open a quantum channel to deep space network!". This should make it possible to have almost realtime monitoring and control of a remote probe with a quantum relay link covering most of the interplanetary distance.

CaioAlonso• 14 years ago

I'm not sure this is FTL communication. I guess this method still requires a conventional communication link.

EDIT: If Wikipedia is reliable on this topic, http://en.wikipedia.org/wiki/Faster-than-light#Quantum_mecha...

pranjalv123• 14 years ago

It's not FTL. Quantum teleportation is like this:

Alice shows Carol and Bob a red ball and a blue ball, and two empty boxes. Without showing Carol and Bob which ball goes in which box, Alice puts a ball in each box and gives one box to Bob and the other to Carol. Carol is an astronaut going to Mars, and they've agreed to open their boxes when she gets there.

Fast forward 6 months later, Carol is on Mars, and she opens her box and sees a red ball. Immediately, without any speed-of-light delay from Earth, Carol knows what Bob will see in his box when he opens it. Note how there's no "useful" information transferred faster than the speed of light - Carol and Bob can't use this to make any useful decisions.

The quantum version is weirder because the balls are in a superposition of red and blue until one of the two opens the box - but there's still no useful information transferred faster than light.

drostie• 14 years ago

What you have described is an EPR experiment, not a quantum teleportation experiment. Also, we don't have any way of persisting a quantum state for 6 months. And, the above discussion is a little misleading because there exists a set of experiments which show "coherence effects" -- basically, that the universe is not obeying the classical probabilities. In this case neither Bob nor Carol see coherence effects on their own qubits (the blue/red state is really a 0-vs-1 distinction, hence quantum bits), but there are coherence effects to be seen in correlations between their measurements -- so if they ever communicate about some experiments they ran, they can do experiments which are inconsistent with the idea that Nature has "made up its mind" whether the ball is red or blue, the same as in double-slit diffraction where Nature has not "made up its mind" whether a photon has gone through the one slit or the other. That's the gist of the EPR experiment.

Let's get into what makes this a teleportation experiment though. Bob and Carol, we assume, still share these EPR pairs of well-entangled objects. What can you do with them? Let me just use "normal" to describe a channel which communicates normal 0's and 1's -- like the Internet does, or radio communications to Mars or what have you.

It turns out, you can communicate one of these clearly-abnormal states from Bob to Carol, using only normal bits. Bob wants to send another quantum state S to Carol but can only send normal bits. He entangles S with his side of the entangled pair, measures the two to get some normal bits, then sends these to Carol. Carol can use the normal bits to modify her own side of the EPR pair into the state S. All of the weird quantum nature of S seems to have "hopped over" sometime between the entanglement and the measurement: the classical information sent just seems too puny, in the sense that we know you can't use a normal channel to communicate a quantum state without such an entangled pair.[1]

Still, the two normal bits that Bob sends seem to be important, in the sense that they tell Carol the right way to "look" at the teleported quantum system, which is not trivial. In that sense there is a much firmer speed-of-light barrier established: the information about the "right way to look at the teleported qubit" is classical, and cannot go faster than light, even though the weird quantum nature of the qubit seemingly did.

[1] That's not entirely true. If Bob makes the state himself then he can tell Carol how to make a copy. Instead you have to imagine that Alice gives quantum states to Bob which Bob doesn't know: Bob can still send these to Carol.

Xcelerate• 14 years ago

Quantum teleportation is pretty cool. But it's one of those things that a lot of people get confused about.

First, what is a quantum state? A "quantum state" is all of the information you need to completely determine a system. If you know a quantum state, then you know everything that there is to know about what it describes. And if we take all of the particles that constitute your body to be "the system", then we can teleport it to a new location. Of course, something as large as a person will probably never happen -- right now we're just doing one particle systems (although a wavefunction can describe as many particles as you want).

The news article says that "matter isn't transported, just the state". This is kind of misleading. There's a property in physics called indistinguishability. Electrons are an example of something that has this property. What this means is that you cannot tell two electrons apart. They are identical in every way (coarsely put, electrons are just an excitation of the same "electron field"). This indistinguishablity leads to some awesome physics (see http://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_statistic...).

So it's meaningless to say that the original matter has or hasn't transported because there's no such thing as the "original matter". If the state of the "new matter" is the same as the old once was, then for all intents and purposes you have actually teleported that object.

But if you have teleported the state, haven't you just cloned the object? Aha! Nature's too tricky to let you pull that off. There's something called the no-cloning theorem which proves that it is impossible to duplicate a quantum state -- you can only move it around.

And to clarify one other misconception, quantum teleportation does NOT allow faster than light information transfer. You still require a classical information channel to complete the teleportation (so it's not instantaneous teleportation in the sci-fi sense).

exch• 14 years ago

This subject has always caused me a great deal of headaches while trying to wrap my mind around it.

If I understand you correctly then, the term 'teleportation' is entirely misleading in this context.

I am also wondering about another issue: Heisenberg's uncertainty principle. How can you accurately gauge the state of a particle, in order to 'transport' it elsewhere, when Heisenberg seems to be convinced this is impossible to do?

Thirdly: 'teleportation' as you explained it, seems to me to be more of a game of "Let's see what's over there, then observe this spot here and wait until some particle randomly displays characteristics similar enough to consider it equal. We can then pretend we did it with science!". Is this the case, or do they actually 'do' something on the other end to create a new particle with a specific state?

Disclaimer: I am very much an arm-chair physicist with as much understanding of this subject matter as the average doormat.

galadriel• 14 years ago

Heisenberg tells you that you cannot measure locations and speed, but Physicist have just accepted it and moved on. What you can measure, and what quantifies a state, are probabilities of things happening. That is all the information that nature gives you, and if you can recreate a set of probabilities from point A at point B, you have teleported the object. The laws of nature ensure that object at point A is destroyed, so you cannot clone states.

The way how you interpret is now philosophy. As far as nature goes, there is no way to distinguish one electron from other, or to differentiate a 'teleported' object from its 'original' object.

locopati• 14 years ago

Your explanation if very clear. Can you recommend a book or two that covers things like this for the layman who has a science background but is not a scientist (i.e. not fluffy, not hardcore)?

juiceandjuice2• 14 years ago

The future of telecommunications, brought to you by Bell's lab instead of Bell Labs.

juiceandjuice2• 14 years ago

One very practical application of this would be for communications to Mars. You could be constantly transmitting a "deferred" signal, only to alter the quantum states when you are ready. We still have a long way to go for ~20 minute quantum state storage times though.

lwat• 14 years ago

You can't use this for FTL communication, sorry.

http://en.wikipedia.org/wiki/No-communication_theorem

Quantum physics at a distance

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