5 ms·

I have never been comfortable with the metaphors that tech reporters use for describing quantum entanglement. They always talk as if it's a spooky interaction

by po 12y ago

I have never been comfortable with the metaphors that tech reporters use for describing quantum entanglement. They always talk as if it's a spooky interaction but the metaphor falls down. Here's an example: I take too halves of an Oreo cookie and send them in opposite directions. Then I use a detector to measure how much filling is on one half and instantly know how much was on the other half. Was that spooky? I understand that at the physics level it's not like this, but the metaphors are never satisfyingly 'spooky' to me as I can easily think up a classical explanation.

stan_rogers 12y ago
That was Einstein's proposal, and it turns out not to be the case, I'm afraid. Claussen's experiment suggested that there's more to entanglement than delaying observation of mutually-dependent properties. Quantum teleportation seems to sew it up, but for that niggling little "were we forced to do what we did" doubt.
- trhway 12y ago
  Interesting that Einstein's proposal with modified Malus's law for single photons produces a "local realism" explanation for the observed apparent Bell's violation in photon polarization experiments. At the Einstein's time they didn't have single-photon counters to confirm/deny, and today it seems that single-photon Malus looks like the classic one (though i didn't find articles directly confirming it, only other articles mentioning it as confirmed). Nevertheless, possibility that such reasonably looking variation of the pretty empirical law produces "local realism" explanation makes me believe that "local realism" battle is far from lost yet :)
kaoD 12y ago
I feel I'll never get to understand entanglement (I hope it's like monads :P). If it's not "spooky" but just half an Oreo... then what is so special about it and why would physicist study it then?
- stouset 12y ago
  The difference is that when you separate a quantum Oreo, until you actually observe which side has the creme on it, the universe hasn't decided yet. Once you observe one half, the universe instantaneously picks one of the two halves at random to contain creme, and one to be empty. We know it must be this way due to statistics. However, that's the point where the Oreo analogy breaks down.
  - FranOntanaya 12y ago
    How do we know the universe hasn't decided?
    - stouset 12y ago
      You and I decide to test whether or not the universe has decided. We have Nabisco mail us quantum oreos, three at a time and we each choose randomly to measure one of them (this is analogous to measuring the spin of an electron in a randomly-chosen axis). Now, if we're measuring them three at at time, there are only two plausible possibilities: one of us has all three cremes (this happens 2 out of every 8 trials) or one of us has two cremes (this happens 6 out of every 8 trials). If one of us has all the cremes, it doesn't matter which one each of us independently chooses to measure; we will both have the same outcome. If one of us has only two of the cremes (e.g., [M]e has two and [Y]ou have one), we agree 5/9ths of the time M M Y M 1 1 0 M 1 1 0 Y 0 0 1 If we run this experiment many, many times, we should converge upon (2/8 * 9/9) + (6/8 * 5/9) = 2/3 of the time our measurements agree. This would be consistent with a model of "preprogrammed" quantum Oreos. This is not what we see in practice. When we actually run this experiment, our measurements agree exactly 50% of the time which can only be explained if the quantum Oreos are not preprogrammed.
- Xcelerate 12y ago
  I have never found a good, simple analogy. The best I've got is still a mathematical explanation, but I think it can make it "click" for some people: You have two photon detectors that you can rotate; one is in New York and one is in Paris. Two photons are entangled and one is sent to New York for measurement, and one is sent to Paris for measurement. What you find is that while each photon's polarization measurement is random, if you take the "random" values you get for both and compute their correlation over many trials, this value is a function of the difference in angles between the photon detector in New York and the photon detector in Paris. But here's the kicker: each photon detector can be rotated while the photons are in flight, before they get there. So how is it that the correlation is a function of both photon detector angles then? Especially considering it is light that you're measuring, and nothing can travel faster than light. That's why it's called spooky action at a distance. (Edit: Let me know if this makes any sense. I'm in a continual search for the simplest explanation of this phenomenon.)
Xcelerate 12y ago
The article was actually written by the MIT professor that's going to be doing the experiment.
yomritoyj 12y ago
J.S. Bell addresses this in the paper https://cds.cern.ch/record/142461/files/198009299.pdf https://cds.cern.ch/record/142461/files/198009299.pdf with the following example: "Dr. Bertlmann likes to wear two socks of different colours. What colour he will have on a given foot on a given day is quite unpredictable. But when you see that the first sock is pink you can already be sure that the second sock will not be pink... " But it turns out that the quantum situation is much spookier.
ivan_ah 12y ago
I have a good analogy for entanglement, which will hopefully help you see the difference between classical (anti)correlations and quantum (anti)correlations. Suppose you want to construct a pair of yes-no-question-answering boxes. The goal is that, however one box answers, the other box will answer the opposite way. The answers produced must be anti-correlated, but I'll just write correlated because it's 4 chars less! Classical correlations are like a pair of boxes that consistently succeed for some specific question. (e.g. repeatedly asking the same question of any one box, will produce 50% yes and 50% no answers, with the other box answering exactly oppositely) Quantum correlations are like a pair of boxes that consistently answer anticorrelatedly for any question. For example, if you ask the boxes the question left-or-right? one will answer left, the other right. If you ask up-or-down? one will say up the other down, front-or-back? --- anticorrelated. And so on for any question. Whether that's spooky, I don't know... To throws some math into this, let's denote |x>, the state of a quantum system, and denote |xy> the joint state of a pair of quantum systems, one in state x, the other in y. Consider the difference between a statistical mixture: 50% of time produce |01> and 50% of time produce [10> and the |Psi-> state which is a 50-50 superposition of |01> and |10>: 1/√2*|01> - 1/√2*|10> a.k.a |Psi-> In both cases if you ask the 0 or 1 question, the answers will be anti-correlated, but you can ask the |Psi-> state any other anti-podal question (e.g. are you |0>+|1> or |0>-|1>?) and the answers will always be anti-correlated. But who has seen this quantum stuff? Isn't |Psi-> just some math scribble on a piece of paper? In fact, |Psi-> is quite real---it is the spin state of the electrons in the unexcited Helium atom. We know there are 2 electrons, and we know they have opposite spins, but we don't know anything else about them. Hence if a He atom were to explode and the electons fly to distant sides of the universe, we still won't know what direction the individual spins are in, the only thing we know is their total spin is 0 -- hence they have opposite spin directions. This is how we get the for any question" aspect of q-entanglement. It really makes sense if you think about these things as information, and q-measurements as answering well posed questions. From the point of view of information, a system is entangled whenever you know more about the system as a whole than about its parts: http://en.wikipedia.org/wiki/Joint_quantum_entropy#Properties http://en.wikipedia.org/wiki/Joint_quantum_entropy#Propertie...
- darkmighty 12y ago
fragsworth 12y ago
You didn't read the article. David Kaiser isn't even really a "tech reporter", he's one of the physicists behind the experiment described in the article, which addresses exactly what you're complaining about.
- po 12y ago
  I think I wasn't clear. I'm not trying to call this one person out for failing here… The tech reporter part wasn't my point. Even physicists don't have very good metaphors to use. My point is that any analogy I've heard fails to convey the strangeness inherit in the underlying math.

by po 12y ago

stan_rogers 12y ago

That was Einstein's proposal, and it turns out not to be the case, I'm afraid. Claussen's experiment suggested that there's more to entanglement than delaying observation of mutually-dependent properties. Quantum teleportation seems to sew it up, but for that niggling little "were we forced to do what we did" doubt.

trhway 12y ago
Interesting that Einstein's proposal with modified Malus's law for single photons produces a "local realism" explanation for the observed apparent Bell's violation in photon polarization experiments. At the Einstein's time they didn't have single-photon counters to confirm/deny, and today it seems that single-photon Malus looks like the classic one (though i didn't find articles directly confirming it, only other articles mentioning it as confirmed). Nevertheless, possibility that such reasonably looking variation of the pretty empirical law produces "local realism" explanation makes me believe that "local realism" battle is far from lost yet :)

kaoD 12y ago

I feel I'll never get to understand entanglement (I hope it's like monads :P). If it's not "spooky" but just half an Oreo... then what is so special about it and why would physicist study it then?

stouset 12y ago
The difference is that when you separate a quantum Oreo, until you actually observe which side has the creme on it, the universe hasn't decided yet. Once you observe one half, the universe instantaneously picks one of the two halves at random to contain creme, and one to be empty. We know it must be this way due to statistics. However, that's the point where the Oreo analogy breaks down.
- FranOntanaya 12y ago
  How do we know the universe hasn't decided?
  - stouset 12y ago
    You and I decide to test whether or not the universe has decided. We have Nabisco mail us quantum oreos, three at a time and we each choose randomly to measure one of them (this is analogous to measuring the spin of an electron in a randomly-chosen axis). Now, if we're measuring them three at at time, there are only two plausible possibilities: one of us has all three cremes (this happens 2 out of every 8 trials) or one of us has two cremes (this happens 6 out of every 8 trials). If one of us has all the cremes, it doesn't matter which one each of us independently chooses to measure; we will both have the same outcome. If one of us has only two of the cremes (e.g., [M]e has two and [Y]ou have one), we agree 5/9ths of the time M M Y M 1 1 0 M 1 1 0 Y 0 0 1 If we run this experiment many, many times, we should converge upon (2/8 * 9/9) + (6/8 * 5/9) = 2/3 of the time our measurements agree. This would be consistent with a model of "preprogrammed" quantum Oreos. This is not what we see in practice. When we actually run this experiment, our measurements agree exactly 50% of the time which can only be explained if the quantum Oreos are not preprogrammed.
Xcelerate 12y ago
I have never found a good, simple analogy. The best I've got is still a mathematical explanation, but I think it can make it "click" for some people: You have two photon detectors that you can rotate; one is in New York and one is in Paris. Two photons are entangled and one is sent to New York for measurement, and one is sent to Paris for measurement. What you find is that while each photon's polarization measurement is random, if you take the "random" values you get for both and compute their correlation over many trials, this value is a function of the difference in angles between the photon detector in New York and the photon detector in Paris. But here's the kicker: each photon detector can be rotated while the photons are in flight, before they get there. So how is it that the correlation is a function of both photon detector angles then? Especially considering it is light that you're measuring, and nothing can travel faster than light. That's why it's called spooky action at a distance. (Edit: Let me know if this makes any sense. I'm in a continual search for the simplest explanation of this phenomenon.)

Xcelerate 12y ago

The article was actually written by the MIT professor that's going to be doing the experiment.

yomritoyj 12y ago

J.S. Bell addresses this in the paper https://cds.cern.ch/record/142461/files/198009299.pdf https://cds.cern.ch/record/142461/files/198009299.pdf with the following example: "Dr. Bertlmann likes to wear two socks of different colours. What colour he will have on a given foot on a given day is quite unpredictable. But when you see that the first sock is pink you can already be sure that the second sock will not be pink... " But it turns out that the quantum situation is much spookier.

ivan_ah 12y ago

I have a good analogy for entanglement, which will hopefully help you see the difference between classical (anti)correlations and quantum (anti)correlations. Suppose you want to construct a pair of yes-no-question-answering boxes. The goal is that, however one box answers, the other box will answer the opposite way. The answers produced must be anti-correlated, but I'll just write correlated because it's 4 chars less! Classical correlations are like a pair of boxes that consistently succeed for some specific question. (e.g. repeatedly asking the same question of any one box, will produce 50% yes and 50% no answers, with the other box answering exactly oppositely) Quantum correlations are like a pair of boxes that consistently answer anticorrelatedly for any question. For example, if you ask the boxes the question left-or-right? one will answer left, the other right. If you ask up-or-down? one will say up the other down, front-or-back? --- anticorrelated. And so on for any question. Whether that's spooky, I don't know... To throws some math into this, let's denote |x>, the state of a quantum system, and denote |xy> the joint state of a pair of quantum systems, one in state x, the other in y. Consider the difference between a statistical mixture: 50% of time produce |01> and 50% of time produce [10> and the |Psi-> state which is a 50-50 superposition of |01> and |10>: 1/√2*|01> - 1/√2*|10> a.k.a |Psi-> In both cases if you ask the 0 or 1 question, the answers will be anti-correlated, but you can ask the |Psi-> state any other anti-podal question (e.g. are you |0>+|1> or |0>-|1>?) and the answers will always be anti-correlated. But who has seen this quantum stuff? Isn't |Psi-> just some math scribble on a piece of paper? In fact, |Psi-> is quite real---it is the spin state of the electrons in the unexcited Helium atom. We know there are 2 electrons, and we know they have opposite spins, but we don't know anything else about them. Hence if a He atom were to explode and the electons fly to distant sides of the universe, we still won't know what direction the individual spins are in, the only thing we know is their total spin is 0 -- hence they have opposite spin directions. This is how we get the for any question" aspect of q-entanglement. It really makes sense if you think about these things as information, and q-measurements as answering well posed questions. From the point of view of information, a system is entangled whenever you know more about the system as a whole than about its parts: http://en.wikipedia.org/wiki/Joint_quantum_entropy#Properties http://en.wikipedia.org/wiki/Joint_quantum_entropy#Propertie...

darkmighty 12y ago

fragsworth 12y ago

You didn't read the article. David Kaiser isn't even really a "tech reporter", he's one of the physicists behind the experiment described in the article, which addresses exactly what you're complaining about.

po 12y ago
I think I wasn't clear. I'm not trying to call this one person out for failing here… The tech reporter part wasn't my point. Even physicists don't have very good metaphors to use. My point is that any analogy I've heard fails to convey the strangeness inherit in the underlying math.