5 ms·

Researchers Advance 'Quantum Teleportation'

55 points by todayiscrown 12y ago

chm 12y ago
I think this is the paper: http://arxiv.org/abs/1401.6958 http://arxiv.org/abs/1401.6958 .
lisper 12y ago
> When Alice measures the state of her photon, Bob's photon changes state as well. NO NO NO! Ten thousand times no! Bob's photon does NOT change state as a result of Alice's measurement. It is in exactly the same state that it was in before: entangled, which is to say, if you consider it in isolation, it is in a quantum mixed state. NOTHING changes on Bob's side as a result of Alice's measurement. It's bad enough that the popular press chronically gets this wrong, but NASA really ought to know better.
- jessriedel 12y ago
  > Bob's photon does NOT change state as a result of Alice's measurement. It is in exactly the same state that it was in before: entangled, which is to say, if you consider it in isolation, it is in a quantum mixed state. NOTHING changes on Bob's side as a result of Alice's measurement. As usual in these situations, it depends on being precise about what one is referring to with words like "state". But with the common definition -- a density matrix updated to incorporate all macroscopic data -- the state of A certainly does change following the measurement of B. It goes from fully mixed to pure. Of course, Bob's personal epistemic state (which doesn't include data that he hasn't yet received from Alive) doesn't change. You can try to say that Alice and Bob's photons have merely become entangled with the measuring apparatus, or some similar many-worlds-type statement, but then you'd be using "state" in a very different way than the vast majority of experimental physicists. You might think that definition better fitting with reality, but it certainly wouldn't make the NASA press release wrong.
  - lisper 12y ago
    It's more than Bob's personal epistemic state that doesn't change. The physical state of his photon doesn't change either. The change from "fully mixed" to "pure" is a change in the mathematical representation of the situation, not a change in Bob's local physical reality. There is no experiment Bob can perform on his photon that will reveal to him whether or not Alice has measured her photon or not (note that this remains true EVEN AFTER he has received Alice's bits). The difference between a mixed and a pure state is simply a difference in mathematical perspective, not a difference in the underlying physical reality.
    - jessriedel 12y ago
      I'm well aware of the physics of the situation. It's my day job. I'm just telling you that NASA hasn't said anything false. The common meaning of "the state of the photon" is exactly as I described above, and it certainly does change. Your discussion of "underlying reality" is besides my point, since those words don't appear in the original article. > There is no experiment Bob can perform on his photon that will reveal to him whether or not Alice has measured her photon or not (note that this remains true EVEN AFTER he has received Alice's bits). I'm not sure what you mean here, but a reasonable interpretation of your words would be false. You consider a situation where Bob has received bits from Alice even when she might not have actually made a measurement. If so then I guess you're saying that Alice just makes these up? If so, then Bob can certainly check to see if Alice has actually made the measurement she claims to have by making a local measurement on his photon, and this can result in an outcome that lets Bob definitively determine that Alice is lying to him. [Of course, sometimes Bob's result agrees with Alice's claimed bits; in this case, Alice has simply guessed correctly for what she will end up receiving in her (now fully determined) experiment.]
      - lisper 12y ago
        > I'm well aware of the physics of the situation. It's my day job. I have met many people for whom this is their day job who nonetheless don't understand this. > Bob can certainly check to see if Alice has actually made the measurement she claims to have by making a local measurement on his photon No, he can't. The best he can do is determine (probabilistically) that the bits Alice gave him are not the result of such a measurement. But he cannot know whether or not Alice's measurement was actually performed. In other words, Bob cannot distinguish: 1. Alice does not perform a measurement, but rather simply pulls two bits out of a hat. 2. Alice does perform the measurement, but sends Bob two bits pulled out of a hat rather than the results of her measurement. He cannot make this determination EVEN PROBABILISTICALLY, i.e. after aggregating many trials all prepared the same way. Because of this, it is wrong to say that Alice's measurement changed the state of Bob's photon. Of course, Bob can distinguish between Alice sending him bits pulled out of a hat and bits resulting from her measurement, in which case he can know that Alice must have performed the measurement. But that's a red herring.
EGreg 12y ago
I've had this conversation on HN and I came to the conclusion there isn't much semantic difference between two entangled photos and, say, two sides of a coin split down the middle. Alice and Bob could have just as easily had two sides of a coin they didn't look at, and the situation would be the same. As soon as you look at what you have, you know what the other person has. If you don't look, but a machine interacts with it, than the same situation arises. The two entangled photons were once close to each other, just like the two sides of the coin. So what's the difference here? Looking at a photon you can't tell whether someone else has looked at the entangled photon. Same with the sides of the coin. http://www.scientificamerican.com/article/quantum-entanglement-creates-new-state-of-matter1/ http://www.scientificamerican.com/article/quantum-entangleme...
- inclemnet 12y ago
  There's a big, very important difference - the quantum state can display non-classical correlations beyond what the classical coin model can describe. We can see this by considering two electrons in an entangled spin state - say, one is spin up, and the other is spin down, but the entangled state means you don't know which is which, only that Alice and Bob will get consistent answers when they measure the spin in the vertical direction. At this point, everything maps fine to the two-half-coins idea, all we know is that they have opposite spins. What's different in the quantum case is that Alice and Bob could instead decide to measure the spin in the left/right direction. Following the rules of quantum mechanics, a given spin up or down state has an undetermined spin left/right state, so when you measure the spin in the left/right direction it has exactly 50% chance of being each one. If the original states were really just like the coin halves, this new measurement would be simply uncorrelated between Alice and Bob - they'd start with different states (up or down), but the left/right measurement would destroy that information and they'd both get a random answer because the individual elecrons end up with an individually random left/right spin direction. The reality is actually different; if we do the measurement maths on the quantum state rather than assuming it's predetermined like the coins, it turns out the left/right spin is still entangled. That means that when Alice and Bob measure the spin in the left/right direction, they'll always find that one of them gets left and the other gets right. This would not be possible if the quantum states were predetermined like the coins. So, the coins analogy is not a bad way to understand some of the basics of what you expect, but it absolutely is not a fully accurate description of what's going on. Maybe you already knew that, but I wanted to be clear that there's very much more to entanglement, because this is the source of several common misconceptions. (Of course quantum teleportation is a further thing again, but the extra non-classical mathematics of entanglement are still important, it's not just coin halves.)

55 points by todayiscrown 12y ago

chm 12y ago

I think this is the paper: http://arxiv.org/abs/1401.6958 http://arxiv.org/abs/1401.6958 .

> When Alice measures the state of her photon, Bob's photon changes state as well. NO NO NO! Ten thousand times no! Bob's photon does NOT change state as a result of Alice's measurement. It is in exactly the same state that it was in before: entangled, which is to say, if you consider it in isolation, it is in a quantum mixed state. NOTHING changes on Bob's side as a result of Alice's measurement. It's bad enough that the popular press chronically gets this wrong, but NASA really ought to know better.

jessriedel 12y ago
> Bob's photon does NOT change state as a result of Alice's measurement. It is in exactly the same state that it was in before: entangled, which is to say, if you consider it in isolation, it is in a quantum mixed state. NOTHING changes on Bob's side as a result of Alice's measurement. As usual in these situations, it depends on being precise about what one is referring to with words like "state". But with the common definition -- a density matrix updated to incorporate all macroscopic data -- the state of A certainly does change following the measurement of B. It goes from fully mixed to pure. Of course, Bob's personal epistemic state (which doesn't include data that he hasn't yet received from Alive) doesn't change. You can try to say that Alice and Bob's photons have merely become entangled with the measuring apparatus, or some similar many-worlds-type statement, but then you'd be using "state" in a very different way than the vast majority of experimental physicists. You might think that definition better fitting with reality, but it certainly wouldn't make the NASA press release wrong.
- lisper 12y ago
  It's more than Bob's personal epistemic state that doesn't change. The physical state of his photon doesn't change either. The change from "fully mixed" to "pure" is a change in the mathematical representation of the situation, not a change in Bob's local physical reality. There is no experiment Bob can perform on his photon that will reveal to him whether or not Alice has measured her photon or not (note that this remains true EVEN AFTER he has received Alice's bits). The difference between a mixed and a pure state is simply a difference in mathematical perspective, not a difference in the underlying physical reality.
  - jessriedel 12y ago
    I'm well aware of the physics of the situation. It's my day job. I'm just telling you that NASA hasn't said anything false. The common meaning of "the state of the photon" is exactly as I described above, and it certainly does change. Your discussion of "underlying reality" is besides my point, since those words don't appear in the original article. > There is no experiment Bob can perform on his photon that will reveal to him whether or not Alice has measured her photon or not (note that this remains true EVEN AFTER he has received Alice's bits). I'm not sure what you mean here, but a reasonable interpretation of your words would be false. You consider a situation where Bob has received bits from Alice even when she might not have actually made a measurement. If so then I guess you're saying that Alice just makes these up? If so, then Bob can certainly check to see if Alice has actually made the measurement she claims to have by making a local measurement on his photon, and this can result in an outcome that lets Bob definitively determine that Alice is lying to him. [Of course, sometimes Bob's result agrees with Alice's claimed bits; in this case, Alice has simply guessed correctly for what she will end up receiving in her (now fully determined) experiment.]
    - lisper 12y ago
      > I'm well aware of the physics of the situation. It's my day job. I have met many people for whom this is their day job who nonetheless don't understand this. > Bob can certainly check to see if Alice has actually made the measurement she claims to have by making a local measurement on his photon No, he can't. The best he can do is determine (probabilistically) that the bits Alice gave him are not the result of such a measurement. But he cannot know whether or not Alice's measurement was actually performed. In other words, Bob cannot distinguish: 1. Alice does not perform a measurement, but rather simply pulls two bits out of a hat. 2. Alice does perform the measurement, but sends Bob two bits pulled out of a hat rather than the results of her measurement. He cannot make this determination EVEN PROBABILISTICALLY, i.e. after aggregating many trials all prepared the same way. Because of this, it is wrong to say that Alice's measurement changed the state of Bob's photon. Of course, Bob can distinguish between Alice sending him bits pulled out of a hat and bits resulting from her measurement, in which case he can know that Alice must have performed the measurement. But that's a red herring.

EGreg 12y ago

I've had this conversation on HN and I came to the conclusion there isn't much semantic difference between two entangled photos and, say, two sides of a coin split down the middle. Alice and Bob could have just as easily had two sides of a coin they didn't look at, and the situation would be the same. As soon as you look at what you have, you know what the other person has. If you don't look, but a machine interacts with it, than the same situation arises. The two entangled photons were once close to each other, just like the two sides of the coin. So what's the difference here? Looking at a photon you can't tell whether someone else has looked at the entangled photon. Same with the sides of the coin. http://www.scientificamerican.com/article/quantum-entanglement-creates-new-state-of-matter1/ http://www.scientificamerican.com/article/quantum-entangleme...

inclemnet 12y ago
There's a big, very important difference - the quantum state can display non-classical correlations beyond what the classical coin model can describe. We can see this by considering two electrons in an entangled spin state - say, one is spin up, and the other is spin down, but the entangled state means you don't know which is which, only that Alice and Bob will get consistent answers when they measure the spin in the vertical direction. At this point, everything maps fine to the two-half-coins idea, all we know is that they have opposite spins. What's different in the quantum case is that Alice and Bob could instead decide to measure the spin in the left/right direction. Following the rules of quantum mechanics, a given spin up or down state has an undetermined spin left/right state, so when you measure the spin in the left/right direction it has exactly 50% chance of being each one. If the original states were really just like the coin halves, this new measurement would be simply uncorrelated between Alice and Bob - they'd start with different states (up or down), but the left/right measurement would destroy that information and they'd both get a random answer because the individual elecrons end up with an individually random left/right spin direction. The reality is actually different; if we do the measurement maths on the quantum state rather than assuming it's predetermined like the coins, it turns out the left/right spin is still entangled. That means that when Alice and Bob measure the spin in the left/right direction, they'll always find that one of them gets left and the other gets right. This would not be possible if the quantum states were predetermined like the coins. So, the coins analogy is not a bad way to understand some of the basics of what you expect, but it absolutely is not a fully accurate description of what's going on. Maybe you already knew that, but I wanted to be clear that there's very much more to entanglement, because this is the source of several common misconceptions. (Of course quantum teleportation is a further thing again, but the extra non-classical mathematics of entanglement are still important, it's not just coin halves.)