5 ms·

German scientists link two labs with ‘universal quantum network’

83 points by po 15y ago

cgs1019 15y ago
Source article: http://www.scientificamerican.com/article.cfm?id=universal-quantum-network http://www.scientificamerican.com/article.cfm?id=universal-q... Source article's source article (Nature mag.; pay wall): http://www.nature.com/nature/journal/v484/n7393/full/nature11023.html http://www.nature.com/nature/journal/v484/n7393/full/nature1... EDIT: arxiv pre-print: http://arxiv.org/pdf/1202.5955v1.pdf http://arxiv.org/pdf/1202.5955v1.pdf
cyanbane 15y ago
The beginnings of the Illusive man.
brainless 15y ago
This is surely genuinely spooky. To quote Arthur C. Clarke: "Any sufficiently advanced technology is indistinguishable from magic."
kator 15y ago
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. :)
- hetman 15y ago
  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.
  - leot 15y ago
    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.
kolev 15y ago
Too bad you can't use this technology to implement an ISP-free Internet. Or... is it possible to have more than two particles entangled?
grannyg00se 15y ago
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?
- gwillen 15y ago
  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.
- deleted 15y ago
  [deleted]
lbo 15y ago
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.
- aik 15y ago
  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?
  - bdonlan 15y ago
    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.
ktizo 15y ago
So, how long now before wintermute merges with neuromancer?
andrewfelix 15y ago
More articles like this please! I find these articles and the informed comments that flow out of them fascinating. More than can be said for some of the questionable articles headlining HN lately.
perlgeek 15y ago
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?
- boxein 15y ago
  This is my understanding as well. I hope someone who knows definitively comes along.
- redwood 15y ago
  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).
- jeffwass 15y ago
  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.
mooom 15y ago
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?

83 points by po 15y ago

Source article: http://www.scientificamerican.com/article.cfm?id=universal-quantum-network http://www.scientificamerican.com/article.cfm?id=universal-q... Source article's source article (Nature mag.; pay wall): http://www.nature.com/nature/journal/v484/n7393/full/nature11023.html http://www.nature.com/nature/journal/v484/n7393/full/nature1... EDIT: arxiv pre-print: http://arxiv.org/pdf/1202.5955v1.pdf http://arxiv.org/pdf/1202.5955v1.pdf

cyanbane 15y ago

The beginnings of the Illusive man.

brainless 15y ago

This is surely genuinely spooky. To quote Arthur C. Clarke: "Any sufficiently advanced technology is indistinguishable from magic."

kator 15y ago

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. :)

hetman 15y ago
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.
- leot 15y ago
  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.

kolev 15y ago

Too bad you can't use this technology to implement an ISP-free Internet. Or... is it possible to have more than two particles entangled?

grannyg00se 15y ago

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?

gwillen 15y ago
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.
deleted 15y ago
[deleted]

lbo 15y ago

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.

aik 15y ago
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?
- bdonlan 15y ago
  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.

ktizo 15y ago

So, how long now before wintermute merges with neuromancer?

andrewfelix 15y ago

More articles like this please! I find these articles and the informed comments that flow out of them fascinating. More than can be said for some of the questionable articles headlining HN lately.

perlgeek 15y ago

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?

boxein 15y ago
This is my understanding as well. I hope someone who knows definitively comes along.
redwood 15y ago
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).
jeffwass 15y ago
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.

mooom 15y ago

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?