The article may be misleading to those not familiar with quantum teleportation. It doesn't have anything to do with instant messages across the galaxy.
"That means it would be possible to build a network of 'quantum repeaters' that use quantum teleportation, rather than photons, to transmit information between different places."
You cannot convey information over distance faster than light using quantum teleportation. You simply ensure that both entangled atoms' wave functions will collapse in a way that's correlated.
Wikipedia has a nice explanation: "Assume that Alice and Bob share an entangled qubit ab. That is, Alice has one half, a, and Bob has the other half, b. Let c denote the qubit Alice wishes to transmit to Bob.
Alice applies a unitary operation on the qubits ac and measures the result to obtain two classical bits. In this process, the two qubits are destroyed. Bob's qubit, b, now contains information about c; however, the information is somewhat randomized. More specifically, Bob's qubit b is in one of four states uniformly chosen at random and Bob cannot obtain any information about c from his qubit.
Alice provides her two measured classical bits, which indicate which of the four states Bob possesses. Bob applies a unitary transformation which depends on the classical bits he obtains from Alice, transforming his qubit into an identical re-creation of the qubit c."
What's cool about this experiment is that it shows you can transmit a qubit's state over distance in a quantum way, which could be useful for a network of quantum computers, which themselves are only useful for a small subset of problems--most involving cryptography. The quantum-repeater application may also be useful in reducing packet-loss-induced error, though it seems to be an incredibly impractical method of doing so for most applications.
Thanks for this explanation. One question: Once the atoms are entangled, I understand that both take on the same superposition. At this point, in any possible way, although we don't know the states, do they actually communicate instantaneously, ie. not at the speed of light?
Put another way -- for the longest time I have believed that through entanglement atoms transmit information instantly (not at the speed of light). Is this in any way true?
It really depends on your interpretation. The math just gives you a continuum of possibilities, each with an amplitude. If you choose to interpret this continuum as being actually real and applying to the entire universe, you don't need spooky action at a distance or FTL travel. You also don't need quantum randomness; the universe always travels down every path of possibility it can, so it never needs to make a random choice. However, this does mean you end up in a 'many worlds' interpretation - but with an amplitude ('measure') attached to each world (corresponding to the square root of the classical probability of that world coming to be). The philosophical implications of this interpretation are quite interesting as well - what happens to all of the 'you's that live in a world in which entropy spontaneously reversed, at an incredibly low measure?
If, on the other hand, you assume that at some point, the universe rolls a die and picks a final outcome, then yes, you do need what is effectively FTL communication of this random roll of the die. This interpretation is easier to understand for many people, since it most closely matches what we think we see. However, it cannot be exploited for transmitting any actual information; the math conspires to ensure the information transmitted is _truly_ random (you provably cannot encode any data under your control onto this channel). You don't even know when the choice is made - indeed, the concept of 'when' the choice might be made here is ill-defined; it may appear to happen after the observation of the result, for some frames of reference.
Is there any difference between these 2 situations?
I have paper with sentence "Earth will be destroyed as of 2012.12.21 24:00 UTC". I am at Andromeda. The truth of the sentence changes from 0.000000001 % (or sth, bear with me) to 0% or 100% instantly, at the moment such time passes on Earth. But this doesn't transmit any information, and I still need to wait for regular light to come from Earth to check how exaclty the truth of this sentence changed.
I'm at Andromeda, I have qubit entangled with qubit on Earth. Someone on Earth changes his qubit, and my qubit changed, but I have to wait for regular bits of information, to be able to read these changes from my qubit.
There is a difference between those two situations. In the many-worlds-ish interpretation, the Earth _never_ goes to either 0 or 100%. Instead, once the moment of reckoning passes, observations of earth return _both_ 'destroyed' and 'non-destroyed' values, entangled together. When you observe this at Andromeda, your own waveform ends up containing "you, having observed Earth's destruction" and "you, having observed Earth not being destroyed" simultaneously. Note that this doesn't require FTL - the combined waveform from earth arrives after however many light-years it takes to get to Andromeda.
The key is, you can never observe that this quantum-mechanical weirdness happened to you, because your point of view only admits a single result at a time. In otherwords, "you" observe everything simultaneously, but each observation happens independently, so you can never think about two mutually exclusive results at the same time.
Nothing that you would consider usable information, referred to as 'classical information', can be transmitted via quantum entanglement. There is 'quantum information' in the state of the entangled particles that can be transferred instantaneously over infinite distance, however attempting the observe this information by any means will destroy it (ie cause the wave function to collapse).
If it was possible to transmit classical information faster than light, this would violate causality as we understand it (you could receive a message that you had been shot before you actually experienced being shot, thus allowing you to prevent yourself from being shot). It's a bit difficult to understand why this is true without a solid understanding of relativity, but you can read up on it a bit here http://en.wikipedia.org/wiki/Special_relativity#Causality_an....
Ditto the Feynman lectures recommendation. Those were incredible icing on the cake of my undergraduate physics course-load, adding significant intuitive meaning and understanding to a subject with much potential but that's taught in a very try manner most of the time.
If you're interested in quantum computation, here's a java simulation I wrote several years ago while a physics grad student. It demonstrates both mathematically and visually the relationship between a qubit spinor and the Bloch sphere, as well as various single-qubit operations.
http://www.pha.jhu.edu/~jeffwass/squankum/index.html
As of now it only supports single-qubit operations (thus no entanglement), as I still needed to figure out an intuitive way to graphically represent two qubits. My idea was to show two Bloch spheres to represent the individual qubits, with a third hemisphere to represent the entanglement, but this is only partially implemented as I'd need to think of the right mathematical relationship for this.
I licensed it under the GPL as project 'Squankum' after being contacted by some software engineers at Computer Sciences Corporation who wanted to see the source code.
For entanglement, which is certainly more interesting than a single qubit, here's another (earlier) java simulation of mine that demonstrates two entangled spins, but it's primarily conceptual, and more removed from the mathematical details.
http://www.pha.jhu.edu/~javalab/entangle/entangle.html
As I only created a github account this week, hopefully this is a chance to demo some of my code for the YC(S12) cycle.
In this scenario, why would Alice and Bob not simply use the mechanism used to transmit the "classical" bits for their entire exchange? What is the value added by the quantum stuff?
You can use fewer photons/electrons/etc to transmit the same amount of information. A qubit's state is a continuum (a superposition) instead of just on or off. If you can entangle qubits with just one photon, you're effectively transmitting a large amount of data with just that one photon plus a little bit of extra information about how Alice's qubit collapsed. This is my understanding of it at least.
It also allows for a much more natural networking quantum computers and synchronization of quantum computer state, without requiring constant translation to-from ordinary bits.
Here's a quote from the Scientific American source article discussing the uses: "Researchers hope that entanglement can be harnessed to circumvent the photon losses that come from passage through optical fibers. In a proposed application called a quantum repeater, a series of nodes, linked by entanglement, would extend the quantum connection down the line without depending on any one photon as the carrier."
So If my understanding of your understanding is correct, this means we can have faster/more efficient data transfer? My question would be can you clarify the "real-world" implications of this?
It's all pretty theoretical at the moment, as is all of quantum computing. No quantum computer with more than a few qubits has been made, and none that are actually faster at any given task than a standard cpu.
It seems that practically this could offer somewhat faster transfers, less packet loss and thus less transfer-time variance, and less bandwidth consumption. This would all be a long way out though, and would require quantum computers in all the routers.
This news is really only interesting insofar as it's one more step toward quantum computing and mastery of the qubit.
So, these answers made sense to me at first, but then I thought some more and now they no longer do.
The problem with the usefulness of the Alice-Bob system is that there are two variable pieces of information: c and the classical bits.
Is there a mechanism with which we can control entanglement with qubit c so that only one agreed upon classical bits key, say 00, is the standard and correct key?
This reduces the Alice-Bob system to only having one variable, c, and enables instantaneous communication at a distance.
If Alice is on Earth and Bob on a spaceship, each having their respective qubits, an entanglement control mechanism, and an agreed upon key, then they can entangle other qubits for communication to their hearts' content and receive that information instantaneously.
I think I need to read more about quantum computing now.
If we figured this out certainly someone/thing way out on the other side of our galaxy or another galaxy has figured it out.. Could it be that we would have to figure out how to DHCP across galaxies and manage to finally share information with a distant intelligence? If so I hope the first thing they send is a cartoon that looks like garfield and we'll finally know we're not alone in the universe. :)
Unfortunately the initial entangling still requires physical contact (provided by proxy of the photon on the optical fibre in this case).
Further more, once entangled, it is still not possible to transmit information (in the way we think of it every day) faster than the speed of light. This would break causality as we understand it.
What quantum teleportation allows is the instantaneous transmission of quantum information between the entangled particles. I.e. the superposition of all possible states can be made the same at both ends. However it is still not possible to know which of these states actually represent the classical information we were interested in. Hence classical information can't go as fast as quantum information.
This is as best I understand it though it would be nice have get an expert to further expand on this.
AFAIK, the reason why entanglement doesn't permit instantaneous information transfer is that, by definition, the quantum state is unknown before you observe it. While, by observing it you instantly know both sides of the entangled pair, this information itself requires conventional communication to the other party in order to be useful, thereby again restricting communication to the speed of light.
Here's something that has always bothered me about this topic:
> That, Ritter says, could extend the application of the network even further: once two atoms are entangled, the quantum state of one depends on the quantum state of the other.
And what exactly is the benefit from that? It sounds as if now you could manipulate one of the atoms, and the other would follow suite, allowing you to transmit information.
But my understanding of quantum physics is that manipulating one atom immediately breaks up the entanglement, so no information transmitted. And this is not just a technical limitation, but a conceptual one.
Am I wrong? And if no, how would a pair of entangled atoms help us in building a communication network?
You're spot on, this is why I find "quantum computing" a bit less exciting than most (just like the "hydrogen economy" say). Both sound great to those who scratch the surface but both are really misnomers. Quantum networks presumably would have a security advantage, but not a speed advantage. Thus there is really not much to get excited about.
Basically entanglement is always dealt with in a misleading way. It's not that one's state influences the other. It's that both are equivalently altered by a third entity limited to the speed of light (as far as we know so far anyway).
Basically, entanglent of individual qubits is an important component of quantum computation. Namely, two coupled qubits have more 'information' and behave differently than two isolated qubits.
Eg, a single qubit can be represented as a 2 element vector (spinor) with complex number elements. That is four Degrees of Freedom (DOF). However, the vector must be normalized, which eliminates one DOF, and the entire vector is phase invariant (can be multiplied by an arbitrary phase exp^(i gamma) for any phase gamma, without effect). So that really leaves two true DOF for a single qubit. These can be represented as angles on the Bloch sphere. See my other post on this thread for a link to my java app to visualize a qubit, and also show the relationship to the spinor vector.
However, a system of 2 qubits
can be represented as a 4 element complex vector. With the same constraints above for single qubits, this leaves 6 DOF for a two-qubit system. Each qubit individually has 2 DOF, but the entanglement itself represents another two DOF!
Many of the quantum computational processes you may have heard about (quantum teleportation, quantum communication, Shor's algorithm) invariably make use of his extra information in multi-qubit systems.
Eg, two spin-1/2 systems can couple together in the spin-0 singlet state, or as a spin-1 triplet state, or as a linear combination of both. This means that two fermions (subject to Pauli exclusion principle) can act like bosons when entangled, and do weird things in aggregate like Bose Einstein condensation. Very funky.
It is explained that there is a photon traveling from one atom (node) to another which encodes information (as quantum state of the atom). Ok.
But then there is a glossing over of what I think is the more impressive part:
"The group also was able to entangle the atoms, which allowed them to share information with each other no matter how far the distance between them. "
So they go from wired-up 60m distance photon information transmission to infinite distance quantum entanglement information transmission. In one sentence. Without any details!
Over what distance did they test this entanglement? Was it as reliable as the wired-up version? Were they able to repeat it under different conditions? Is there a reason they were limited to only two nodes?
Quantum entanglement works over any distance, but can't be used to transmit information. They most likely did not try further separating the atoms; the fact that entanglement is an unbounded-distance effect is just a statement of the physics involved.
I wonder if a SETI experiment can be established by sending information by quantum codification, do we know how to receive or scan for this kind of signals?
"That means it would be possible to build a network of 'quantum repeaters' that use quantum teleportation, rather than photons, to transmit information between different places."
You cannot convey information over distance faster than light using quantum teleportation. You simply ensure that both entangled atoms' wave functions will collapse in a way that's correlated.
Wikipedia has a nice explanation: "Assume that Alice and Bob share an entangled qubit ab. That is, Alice has one half, a, and Bob has the other half, b. Let c denote the qubit Alice wishes to transmit to Bob. Alice applies a unitary operation on the qubits ac and measures the result to obtain two classical bits. In this process, the two qubits are destroyed. Bob's qubit, b, now contains information about c; however, the information is somewhat randomized. More specifically, Bob's qubit b is in one of four states uniformly chosen at random and Bob cannot obtain any information about c from his qubit. Alice provides her two measured classical bits, which indicate which of the four states Bob possesses. Bob applies a unitary transformation which depends on the classical bits he obtains from Alice, transforming his qubit into an identical re-creation of the qubit c."
What's cool about this experiment is that it shows you can transmit a qubit's state over distance in a quantum way, which could be useful for a network of quantum computers, which themselves are only useful for a small subset of problems--most involving cryptography. The quantum-repeater application may also be useful in reducing packet-loss-induced error, though it seems to be an incredibly impractical method of doing so for most applications.