quantom teleportation
August 26, 2009 7:44 PM   Subscribe

Is quantom teleportation instant?

The reason i am asking is because i was wondering if it is instant we could use it for instantaneous data transfer over inter steller distances say from a prob on the edges of our solar system?

or say in 200 years our mars colony is connected to earth by quantom data teleportation ! no more lag just because your on the other side of the planet or solar system !:)

this is the first thing i thought about when i heard about it also if you can teleport light couldnt you also teleport electrons and power something like that ?
posted by quseio to Science & Nature (18 answers total) 1 user marked this as a favorite
Scientific American has an article that describes some of the practicality of these devices:

"Kwiat also sees this work finding applications in quantum communication as a link between quantum processors. But he would like to see the system boosted to higher operating speeds—in the current incarnation it takes an average of 12 minutes, or about 30 million attempts, to secure entanglement between a pair of ions."
posted by Aanidaani at 7:50 PM on August 26, 2009

How many people will beat me to saying that faster-than-light communication is understood to be impossible at the moment?
posted by tss at 7:55 PM on August 26, 2009

I think this question is too speculative for Ask MeFi and should be instead chatfilter, but for what it's worth, you may want to look into so-called "spooky action at a distance", which describes how particles separated by millions of miles can still affect one another in realtime.

Take this with a grain of salt, as I'm a humanities/philosophy geek rather than a physics geek, but to the best of my knowledge no one can really say what quantum data transfers will or would be like yet. Much of the work being done is still heatedly debated. For example, there is a decent argument that states that quantum computing is basically unusable, because the state of the data inherently changes when the waveform collapses under observation, nullify most of the real utility provided by the mechanisms of data manipulation; equally, there is a counter-argument that quantum entanglement can provide an out by allowing for indirect stateful observation of entangled particles.
posted by ellF at 7:58 PM on August 26, 2009

Actually, this page is better at glossing over why quantum teleportation can't give you faster-than-light communication.
posted by tss at 8:01 PM on August 26, 2009

Sorry, it took me two minutes to secure the entanglement of the submit button.
posted by furtive at 8:04 PM on August 26, 2009 [5 favorites]

That would be instantaneous in the temporal and simultaneous in the quantum. Inside the light cone instantaneous, outside the light cone simultaneous.
posted by hortense at 8:38 PM on August 26, 2009

Information cannot be sent at the speed of light. One of the steps of quantum teleportation necessarily involves sending information to the distant particle, usually in the form of a photon, and thus tends to happen exactly at the speed of light.

Bell's Inequality type events do happen instantaneously, but it can be shown that this does not actually transmit information. So if you measure one side of two entangled particles, it does affect the other instantly, but such a thing doesn't actually let the other side know anything new. Alas.
posted by Schismatic at 8:56 PM on August 26, 2009 [2 favorites]

Define "instant". "Instant" relative to what...your perception?
posted by hal_c_on at 10:45 PM on August 26, 2009

How many people will beat me to saying that faster-than-light communication is understood to be impossible at the moment?
There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, seven hundred and fifty miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it the sound barrier.
At the moment.
posted by kirkaracha at 11:16 PM on August 26, 2009 [1 favorite]

Schismatic has it; the point is that one of the characteristics of quantum teleportation is that it doesn't transfer information.
posted by koeselitz at 11:59 PM on August 26, 2009

Er - sorry. I mean that the point is that quantum teleportation is only of information, and that it isn't subluminal.

"Teleportation" is actually probably a pretty bad word for it; it's actually more akin to switching tangled and unentangled particle as you might switch ions in a wire to create electricity. It takes a long time, longer the further you want to do it, and it doesn't achieve anything faster than what we already have.

It's probably faster than USB 2.0, though.
posted by koeselitz at 12:13 AM on August 27, 2009

Schismatic has it; the point is that one of the characteristics of quantum teleportation is that it doesn't transfer information.

False. It just doesn't do so faster than light since classical communication is part of the protocol.
posted by fatllama at 1:14 AM on August 27, 2009


Lets say you want to convey some information to me with entangled particles. You entangle the particles, send one to me and keep the other.

That particle can't reach me faster than the speed of light, so the information you're sending me, entangled or not, can't reach me sooner. It's really no different than me flipping a coin. I don't look at the coin, but I FedEx it to you to look at. The information already exists, but no one knows what the information is until someone looks at it. But you can't look at it until it arrives, and it can't arrive faster than light.

Hm. Re-reading that I'm no sure I made it any clearer.
posted by Ookseer at 2:20 AM on August 27, 2009

I think the game Portal illustrates why teleportation can't happen, at least not how depicted in Portal. If you set up one portal on the ceiling and one on the floor, you can fall forever. Because you are under constant acceleration, your speed increases without bound. In particular, it increases beyond the potential energy you had when you started falling. SECOND LAW VIOLATION!
posted by DU at 5:10 AM on August 27, 2009

From the wiki:

"The protocol has three steps: measure a and b jointly to yield two classical bits; transmit the two bits to the other end of the channel (the only potentially time-consuming step, due to speed-of-light considerations); and use the two bits to select one of four ways of recovering c."

Two points I know to be true:

1) You don't have to transport entangled particles at the moment of requirement, but they are typically entangled (initialized) when close together. They will stay "special" as long as you can keep them from interacting with the environment (the toughest challenge in the field today); we have no reason to believe the distance between them matters.
2) You do have to transmit bits after you've chosen to teleport. This means you'll be constrained by how fast you can send someone information (proved in all areas of physics to be limited by the speed of light).
posted by gensubuser at 5:50 AM on August 27, 2009

Information cannot be sent at the speed of light. One of the steps of quantum teleportation necessarily involves sending information to the distant particle, usually in the form of a photon, and thus tends to happen exactly at the speed of light.

Whoops, screwed up the first sentence quite a bit there. Information cannot be sent FASTER than the speed of light. At the speed of light is easy. And yes, just as fatllama said, part of quantum teleportation is sending an entangled photon from point a to point b in the good ol' fashioned classical way. That's what I meant by it happening at exactly the speed of light. Quantum teleportation is cool because of its control and fidelity and is thus often looked at with regards to quantum computation, but nothing in physics suggests that we're going to get over this pesky speed of light barrier ever.
posted by Schismatic at 6:13 AM on August 27, 2009

This is a good question that people get paid to think seriously about it. The short answer is that nobody knows, but probably not.

First to dispel a misunderstanding: quantum teleportation does not involve the transfer of matter from one place to another, like the transporter on "Star Trek" did. There's no way to make electrons in a region of space where there aren't any (unless you also make antielectrons, or neutrinos). It might be possible to "teleport power," in some sense, but this would be terrifically inefficient, like using an aircraft carrier to deliver a stick of firewood.

What's teleported in entangled quantum systems are correlations. The classic example is a pair of particles whose spins are opposed: the "north" pole of one points along the "south" pole of the other. If I measure particle A and find its north pole points up, I know that whoever measures particle B will find the north pole pointing down. But, because of the funny way spins work in quantum mechanics, the person who measures B can ask whether its north pole points left or right. My measurement tells me nothing about that direction. If the other experimenter and I compare notes, we will have correlated results for the entangled pairs where we both asked "up or down", and uncorrelated results where one asked "up or down" and the other "left or right." But it's not possible to see the correlation without this classical, subluminal transfer of information.

It was first shown in the 1980s by Aspect and collaborators, and many times since, that these correlated measurements work even when the choice between "up or down" and "left or right" is made independently at each end, fast enough that observers in different reference frames could argue about which measurement came first. (Look for "spacelike separated double delayed choice".) There was a Nature paper in the past two or three years where the two measurements were carefully made simultaneous in the reference frame of the earth; their conclusion was that, if relativity were wrong and some signal did travel from the first measurement to the second, the signal would have to travel more than ten thousand times faster than light. However, a criticism of all these experiments is that the entire decision process (generating a random number, moving a mirror or polarizer, interacting with one entangled particle, recording the measurement result) takes at least several hundred milliseconds. An ordinary luminal signal could cross the earth several times in this interval. So maybe no one has done a real delayed choice experiment after all. There's an unexpected reason to go to the moon.

So if that's the evidence, why do I think that quantum teleportation is "probably not" instantaneous? It's because the usual description is only semi-quantum: the correlations are not present until you include a classical information transfer. There are quantum-mechanical descriptions for the detection process and the information transfer, too, though for a real system they become unworkably complex. The correlation isn't present until the entire measurement is complete. The intermediate state where there seems to be extra information suggests (to me) a global symmetry that includes the behavior of the detector rather than a transition that's forbidden by the description of the interaction.
posted by fantabulous timewaster at 12:58 PM on August 27, 2009

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