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Scientists Teleport Information Between Ions a Meter Apart
erickhill writes with word that scientists from the University of Maryland have successfully transferred information from one charged atom to another without having it cross the intervening space of about one meter. The academic paper is available in the journal Science, though it requires a subscription to see more than the abstract.
Scientists have previously teleported unmolested qubits between photons of light, and between photons and clouds of atoms. But researchers have long sought to teleport qubits between distant atoms. Light's high speed of travel makes photons good transporters of information, but for storing quantum information, atoms are a much better choice because they're easier to hold on to. 'This is a big deal,' comments Myungshik Kim, a quantum physicist at Queen's University Belfast in the United Kingdom. 'To store information as it is in quantum form, you have to have a teleportation scheme available between two stationary qubits. Then you can store them and manipulate them later on.'"
"teleportation" always seems to lead people to the wrong conclusions. This is about transferring the informational content of a qubit. Which you can't perfectly represent with a classical system. I can see how this as the one commenting physicist claims is a "big deal" when it comes to building quantum computers. But it's not about instantaneous matter transport or superluminal communication.
I'm not sure what the article meant by ultra secure "quantum communication". Quantum teleportation *is* a quantum communication *channel* but it's unclear what kind of security they are talking about. Perhaps "Quantum Encryption" but that's another term that often sends people down the wrong track.
Say particles A and B are entangled, and you are in a position to observe B, but not A. You have no way to know whether A has already been observed, because B will look the same to you either way, unless you already know the state of A.
you have both a black and red marble and you send one around the world, well when one guy checks and sees that his marble is red, the other guy instantly knows that his marble is black.
More to the point, the other guy can find out his marble is black, but only if you communicate to him that your marble was red. Thus information was transferred, but you have to communicate by other means to make it meaningful, which defeats the purpose. It's like sending someone an encrypted message over an insecure channel. Great until you realize you now have to send him the key over the same channel. Sure it's encrypted, but the means of making it useful renders it ineffective.
I work for the Department of Redundancy Department.
Okay, can you clarify for me why exactly you can't? Is it because you can't actually control what state the measured atom, and thus the distant atom, will take?
Sure, I'll try: A quantum 'entangled' state means that two systems are in an 'undefined' state in the quantum sense, that are interdependent.
When one is measured, the other one will _instantanously_ adopt whatever state is 'required' to complement the other one. So one 'knows' instantly what the other is doing, so to speak. Which means a sort of information has been transferred at FTL speed.
The reason why this can't actually be used for communication is twofold: One is exactly as you said: Because you can't know which state you'll measure, you can't transfer information through that alone. The second reason is that, an entanglement between two systems occurs only if there's an (unmeasured) interaction between them.
That means you either separate the two systems from each other (as in the classic example of entangled photons moving apart), or as in this case, by letting them interact with photons - that travel at light speed. Either way though, light speed is the best you can do.
Yeah. I will try to give a simplified explanation to non-experts (I'm just a curious guy myself):
First you entangle two particles. Then you let one travel somewhere. (If at bumps into another particle on that way, the particle loses the entanglement.)
Now if you "measure" the first particle, the "wavefunction" (the entanglement) of both particles collapses in a specific way.
By measuring that traveled particle, you can get the information on how the other particle got manipulated when it lost the entanglement.
The nice thing about this is, that it is instantly. There is no measurable delay.
So you could theoretically entangle a ton of material with another ton of material, and then send the first ton up to some remote planet. (Which of course would take very long. But you could send it at very high speeds which no human could survive too. For example by using a rocket that uses nuclear explosions as propulsion.)
Say you have defined, that you can use 0.5 kg of material every year for each side, and split the ton in such "blocks". Then you just write the outgoing 0.5 kg block (you collapse the entanglement) over the year, and read the incoming 0.5 kg block at the end of every year. By using a special encoding, you can detect where the data ends, and where the data collapsed trough your measurement. Or you just pipeline the to-be-written data on both sides, and read at the end of every month, week, day, hour, minute, second... whatever is most reasonable. (Making it a buffered transfer of blocks.)
This would give you a thousand years of infinite-speed (depending on your read rate) communication with the bandwidth of 0.5 kg of material per year (~1,37 g per day). (The amount of bits depends on the material.)
Any sufficiently advanced intelligence is indistinguishable from stupidity.
You can't determine if a particle is in a super-position or not, because any measurement of it will instantly collapse the waveform on both particles, and if you collapse yours first you will be unable to receive the information being transmitted by the other. You will need to know that the other entangled particle has already been collapsed, before you read yours, and that information still has to get to you by a conventional method.
http://www.mhall119.com
Is there any way to know that measurement has taken place at the other end and your local qubit has collapsed?
Crash course in quantum mechanics, perhaps this explains it: a binary quantum mechanical system is in a linear superposition of states A and B. That is, it is either 100% A, or 100% B, or anything in between; for example 70% A and 30% B.
Now if you measure, you would only get "pure" results, i.e. purely A or purely B. If the system was pure (i.e. 100% B) before the measurement, you get what it was. If the system was mixed (say, 70-30), and you had the chance to measure the system more than once, then you get A in 70% of the cases, or B in 30%. For example: make 1000 copies of the system, and measure each of them. Roughly 700 (give/take a few) would be A, roughly 300 would be B.
The biggest problem is that you don't have 1000 exact copies -- unlike with classical information, basic QM forbids cloning of a system. So you basically have one shot, and if you happen to measure B, you'll never know whether it was because of a 100% pure B state, or simply because you "got lucky".
I mean, I know the answer is you can't communicate instantly, I'm just figuring out why (mostly to help explain to people with roughly my same layman's understanding of physics why instant communication is impossible).
While the "quantum information" is being transfered instantaneously, the problem is that the quantum state is not transfered 1:1 onto the target. It is ... "twisted". Imagine that like x*A+y*B (-> teleport ->) y*A+x*B. Now you know that the numbers x and y mean the same in both systems -- you just don't know exactly how they would be twisted after the teleportation. There are 4 possibilities how they can be twisted, and all 4 are equally probable, there's nothing you can do to favor the one over the other.
However, after the teleportation, the guy at the source can tell how they have been twisted (because the teleportation act itself is a measurement, which's result tells him exactly what happened), but the guy at the target does not.
So at first, even if the guy at the target knows that the atom has been "teleported", he stil doesn't know which one of the 4 twisted flavors of the original atom he got. If he just takes a "wild guess" and tries to measure, he'll get a statistical result which reveals absolutely no information about the actual coefficients.
The target-guy needs the source-guy to tell him which of the 4 twists occured, or in short: needs an information transfer in order to be able to "untwist" his atom and have an exact copy.
Again, the important part is that if the target-guy does not "untwist" his atom, but instead decides to go away and measure it anyway, he'll have an overall chance of 50-50 (regardless of the original x and y) to measure either A or B, so there's no information whatsoever that he could gain, not even from repeating the experiment.
It's the "twist" that makes the twist with teleportation... :-)
Here's an illustration of the non-tranmission of information via entanglement.
Suppose we have a pair of 'magic coins'. Either coin can be flipped and come up either heads or tails, and the other coin will always come up the opposite.
Now, suppose 2 people meet in New York and agree that they will meet again in Oslo if Amy's coin comes up heads and Bill's coin comes up tails, or they will meet in Sidney if Bill's coin comes up heads and Amy's coin comes up tails. Then Amy goes to Peking and flips her coin. It comes up heads, so she meets Bill in Oslo.
The information, which city they will meet in, was AGREED ON BEFORE HAND, it wasn't 'transmitted' by the flip of the coins. The information was in Amy's head when she went to Peking, it traveled by a classical channel governed by relativistic limitations.
This can be seen explicitly if you assume that Amy and Bill DIDN'T agree on which face of the coins meant Oslo or Sidney. In that case when Bill and Amy flip their coins they DO know that their opposite number's coin came up the other way, but neither of them knows which city to go to! In other words, no information was conveyed between them BY the flip of the coins.
"Malo periculosam, libertatem quam quietam servitutem." -- Jefferson
A completely valid arguement-- until 1964:
http://en.wikipedia.org/wiki/Bell's_theorem