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Is Quantum Entanglement Real?

99 points by dctoedt 12y ago

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.
softdev12 12y ago
Even with experiments still ongoing, quantum entanglement is most likely real. I think David Kaiser wouldn't suggest otherwise. There's a real nice section in wikipedia re: the testing of quantum entanglement (see http://en.wikipedia.org/wiki/Quantum_entanglement http://en.wikipedia.org/wiki/Quantum_entanglement) "Systems which contain no entanglement are said to be separable. For 2-Qubit and Qubit-Qutrit systems (2 x 2 and 2 x 3 respectively) the simple Peres-Horodecki criterion provides both a necessary and a sufficient criterion for separability, and thus for detecting entanglement. However, for the general case, the criterion is merely a sufficient one for separability, as the problem becomes NP-hard.[59][60] A numerical approach to the problem is suggested by Jon Magne Leinaas, Jan Myrheim and Eirik Ovrum in their paper "Geometrical aspects of entanglement".[61] Leinaas et al. offer a numerical approach, iteratively refining an estimated separable state towards the target state to be tested, and checking if the target state can indeed be reached. An implementation of the algorithm (including a built in Peres-Horodecki criterion testing) is brought in the "StateSeparator" web-app."
jmcminis 12y ago
It seems the assertion is that there are 2 options, 1. quantum entanglement exists 2. "Some unnoticed causal mechanism in the past may have fixed the detectors’ settings in advance, or nudged the likelihood that one setting would be chosen over another." The human brain may or not be deterministic, but it is certainly a complex system. For the results to be so consistent for the multitude of experiments that have been performed, it seems highly unlikely that #2 is the case. I think in this case it's pretty fair to apply Occam's razor. On a side note, I hate titles for popular physics articles. The next article titles: "5 super simple steps to splitting the atom" "20 crazy tips for growing high temperature superconductors" "Make free cash farming Higgs bosons"
- snowwrestler 12y ago
  Option 2 is similar to the Novikov self-consistency hypothesis of time travel, which states that if an action would cause a time travel paradox, its probability is zero. Under this hypothesis, even if entanglement does allow changes to move faster than light--essentially travelling backward in time--any action that would expose this time travel would be forbidden. The scientist is physically prevented from making the "wrong" choice when setting the detector. The problem with this hypothesis is that it seems essentially untestable. Another way to think of option 2 is that the universe is 100% deterministic. Under this hypothesis, the speed of entanglement is only apparent, not real--like planning with a friend to both turn on your flashlights on opposite hilltops 2 hours from now. Without knowing that it was planned in advance, an observer would wonder how you reacted to one another so fast. This is also impossible to fully test; at best you could prove that the consistency extends as far back as we can see into the past. And that is exactly what these scientists will be testing.
  - jmcminis 12y ago
    The Novikov self-consistency hypothesis is pretty neat. I like the thought of the researcher travelling back in time to create the apparatus such that the result always confirms quantum entanglement. It's classical and not quantum so it's not clear to me how it relates. I'm not sure anyone (besides philosophers) takes the loopholes seriously.
bhouston 12y ago
Some questions: - Can we use this to send large amounts of data around the world quickly? Can it replace intercontinential data links? - Can we use this for interstellar communication channels of limited amounts of data? - Can we use this to pass messages between two locations in a way that is immune to easedropping? - What are other real-world applications of this?
- onewaystreet 12y ago
  Unfortunately communication through QE isn't possible: http://en.wikipedia.org/wiki/No-communication_theorem http://en.wikipedia.org/wiki/No-communication_theorem
  - Xcelerate 12y ago
    But you can use it to double your bandwidth (http://en.wikipedia.org/wiki/Superdense_coding http://en.wikipedia.org/wiki/Superdense_coding) and "store" bandwidth (http://strilanc.com/quantum/2014/05/03/Storing-Bandwidth-with-Superdense-Coding.html http://strilanc.com/quantum/2014/05/03/Storing-Bandwidth-wit...)
- pontifier 12y ago
  From my understanding, it cannot be used to transmit information because the measured property of the entangled photon or particle is not chosen by the the experiment. It is truly random. It could be used to ensure that 2 locations have the same random data. This is useful in cryptography.
- beloch 12y ago
  It's a very common misconception that entanglement allows faster than light communication. It doesn't. Describing quantum phenomena with real world metaphors always breaks down if we carry things far enough. I'm going to use coin flipping as a metaphor to make a specific point, but don't assume this metaphor will still work if you try to extend it. To even begin to understand entanglement you really need to look at the math. That being said, let's proceed. The Setup: (this explains how the metaphor is being made, but you can skip to the payoff if you just want the what without any of the why). We can think of a pair of entangled particles as a pair of coins. If we flip each coin, we have a fifty/fifty odds of gettings heads or tails. This is what we normally think of, in the macroscopic world, as a random outcome (It's not really, but let's not go there). An entangled pair of coins behaves very unusually. The outcome of flipping one coin is related to the outcome of the other coin. For example, if one coin comes up as heads, the other will always come up as tails, or vice-versa. Let's say we separate the two coins by several light years and then flip them. The strange correlation will hold. For a non-Quantum particle there are several possible explanations for this behavior. We might think that, somehow, the measurement of one particle causes the outcome of the other article to change. However, time depends on reference frame. If the two coins are in relative motion to each other in the right manner, which particle was measured first is relative (There is no universally preferred reference frame). i.e. One observer might correctly say that the first coin was measured before the second, but another observer might see things reversed. So, which measurement is the cause and which is the effect? Another notion is that the outcome was preordained when the coins were made. I'd have to go into the math a bit to adequately explain this, but with entangled quantum particles you can create experiments that demonstrate this is impossible. Tests of Bell inequalities are especially notable for this. This is Einstein's "spooky action at a distance", a behavior exhibited by entangled particles. The Payoff: So, if a measurement performed on two entangled particles is like flipping a pair of coins, how can we use that to communicate. Well, we can't. Not directly. You see, we can't force a coin to come up heads or tails. If we look at the outcomes of just one coin they will always appear random and uncontrollable. It's when we talk to the guy with the other coin that we'll say, "Hey, I knew what you were going to get because of what I got!". So, classical communication, limited by the speed of light, is necessary to figure out what these correlations are. So forget about FTL communication. However, what about communicating secrets? Well, that's a whole other kettle of fish! It turns out quantum correlations are fantastic for communicating secret keys. Google quantum cryptography. As for other applications, google quantum computing. Entanglement is absolutely intrinsic to this field. There are also applications in medical imaging and metrology (i.e. standards of measurements).
Xcelerate 12y ago
All components of a quantum system are always entangled. It's not some mysterious property that arises under unusual circumstances. In fact, I would argue that quantum entanglement is the defining feature of quantum mechanics that distinguishes it from classical mechanics. However, the way that most articles (and even physicists) use the term "entanglement" is in a loose way. Within a good approximation, a multi-component quantum state can be represented as a tensor product of single component quantum states. When this isn't a good approximation anymore, you can say that the entanglement of the system is much more apparent. To give an example of what I mean, there's a technique in quantum chemistry known as density functional theory (DFT), which is used to compute ground state energies of various molecules. For some molecules, it works pretty well. The benefit of DFT is that it is a fast calculation technique (well, as far as quantum chemistry goes), but it's speed comes at a price. Rather than using the Coulombic interaction of every pair of electrons to compute the energy of a molecule (or more technically, using every Coulombic interaction as terms in the Hamiltonian), a probability cloud of electron positions is computed instead, and this cloud is used as the energetic term. This works really well for a lot of systems, but in some cases (superconductors, metals, solid-state physics, van der Waals interactions), this approximation falls apart because the effect of electron-electron correlation significantly affects the energy of the system. In this case, the full, non-separable wavefunction is required. The fact that this full wavefunction cannot easily be broken into an approximation of simpler, single electron wavefunctions means that the entanglement of the system is very apparent.
- orbifold 12y ago
  For someone who is more of a mathematician than physicist, the following article might be of interest in this context: http://math.ucr.edu/home/baez/quantum/node4.html http://math.ucr.edu/home/baez/quantum/node4.html The gist is that the state space of a multicomponent quantum mechanical system (think electrons in a metal) is given by the tensor product of the single component state spaces. As might be familiar from linear algebra, not every element in the tensor product of two vector spaces is simply a tensor product of two vectors. That's really all there is to entanglement, but unfortunately you can't explain that to a lay person.
- gus_massa 12y ago
karmakaze 12y ago
I see nothing counter-intuitive about quantum entanglement. Photons travel at the speed of light--time does not exist for them. Alter one, the entangled one is instantaneously connected.
- readerrrr 12y ago
  That does not work because light speed has a limit. It is true that the photon does not experience time. Imagine a photon traveling a distance of 1 light year towards us( we are stationary). In it's reference frame it will do so immediately but in our it takes 1 year. For the photon to travel to us in zero time in both frames, the speed of light would have to be infinite, which we know it isn't.
- krapp 12y ago
  "If you think you understand quantum mechanics, you don't understand quantum mechanics." - Richard Feynman
  - readerrrr 12y ago
    I wish you comment were downvoted instead of karmakaze's one. He is expressing an opinion and even if he is wrong, it is still an interesting idea, and shows some effort. A decent short explanation could show the mistake, and make a good comment->answer combo. Instead you quoted a comment which is out of context ( Feynman's comment is intended for physicists of his level ), contributing nothing. ( Please don't take this as a personal attack ( it isn't), but more a thought what's wrong with HN in light of recent debate )
    - krapp 12y ago
      No, you're right - it was snide and I should have reconsidered it.
- jostylr 12y ago
  The issue is with instantaneous. What does that mean in a relativistic context? Depending on the speed of the observer, "instantaneous" changes. This is fine in reconciling some story in the future, but if there is an action happening instantaneously with something else, then we need a notion of same instant. This is easy to add to have something working, but it is extremely hard to add it in a principled way.
danbruc 12y ago
Good opportunity to again promote »The Theoretical Minimum« [1] by Leonard Susskind (born June 1940, director of the Stanford Institute for Theoretical Physics with research interests in string theory, quantum field theory, quantum statistical mechanics and quantum cosmology). It is pretty accessible and a key takeaway is that there is a lot of misinformation in the wild and especially on the internet when it comes to (modern) physics as he tell his class not only once when someone asks about something he read somewhere. [1] http://theoreticalminimum.com/ http://theoreticalminimum.com/
- paladin314159 12y ago
  This is really cool. I took a (very) basic class on modern physics with Susskind; awesome guy and great at explaining things. Will have to read through this!
  - danbruc 12y ago
    Watch, not read, these are video recordings of courses. About 20 courses with about 10 lectures each, usually about 2 hours.
- gus_massa 12y ago
  The site looks interesting, but it's difficult to find the information that is related to the discussion. Does this site have a page that discusses specifically entanglement? EPR paradox? Bell inecuality? I'd like to see a link to the specific relevant page and not a link to the general site.
  - danbruc 12y ago
    Here is the course on quantum entanglement [1] but depending on prior knowledge it may be wise to first go through the courses on classical [2] and quantum [3] mechanics. He tries to keep everything as self-contained as possible but does not go through the very basics things like Lagrangians and Hamiltonians every time. [1] http://theoreticalminimum.com/courses/quantum-entanglement/2006/fall http://theoreticalminimum.com/courses/quantum-entanglement/2... [2] http://theoreticalminimum.com/courses/classical-mechanics/2011/fall http://theoreticalminimum.com/courses/classical-mechanics/20... [3] http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter http://theoreticalminimum.com/courses/quantum-mechanics/2012...
jaekwon 12y ago
I assume that photons act as waves and not particles. Though they have quantized energy (frequency), there's no reason for me to think that they have locality. If so... The most plausible explanation I can think of is a Monty Hall paradox at the source of the emitter. Thousands of photon pairs? How are they generated? Is their emission dependent on the destination via say, pilot wave resonance? Either resonance or waves that travel backwards in time. Experiments of entanglement speed should point towards one or the other.
Ih8registering 12y ago
Good to see the hoax of quantum superstition (superposition) is in its last days. People are wising up!
- flatestcat 12y ago
  Yeah, there's less rationale for BS like quantumajickalism as grants dry up. That's all this made up BS was designed for --- to scam grant money.

99 points by dctoedt 12y ago

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.

softdev12 12y ago

Even with experiments still ongoing, quantum entanglement is most likely real. I think David Kaiser wouldn't suggest otherwise. There's a real nice section in wikipedia re: the testing of quantum entanglement (see http://en.wikipedia.org/wiki/Quantum_entanglement http://en.wikipedia.org/wiki/Quantum_entanglement) "Systems which contain no entanglement are said to be separable. For 2-Qubit and Qubit-Qutrit systems (2 x 2 and 2 x 3 respectively) the simple Peres-Horodecki criterion provides both a necessary and a sufficient criterion for separability, and thus for detecting entanglement. However, for the general case, the criterion is merely a sufficient one for separability, as the problem becomes NP-hard.[59][60] A numerical approach to the problem is suggested by Jon Magne Leinaas, Jan Myrheim and Eirik Ovrum in their paper "Geometrical aspects of entanglement".[61] Leinaas et al. offer a numerical approach, iteratively refining an estimated separable state towards the target state to be tested, and checking if the target state can indeed be reached. An implementation of the algorithm (including a built in Peres-Horodecki criterion testing) is brought in the "StateSeparator" web-app."

jmcminis 12y ago

It seems the assertion is that there are 2 options, 1. quantum entanglement exists 2. "Some unnoticed causal mechanism in the past may have fixed the detectors’ settings in advance, or nudged the likelihood that one setting would be chosen over another." The human brain may or not be deterministic, but it is certainly a complex system. For the results to be so consistent for the multitude of experiments that have been performed, it seems highly unlikely that #2 is the case. I think in this case it's pretty fair to apply Occam's razor. On a side note, I hate titles for popular physics articles. The next article titles: "5 super simple steps to splitting the atom" "20 crazy tips for growing high temperature superconductors" "Make free cash farming Higgs bosons"

snowwrestler 12y ago
Option 2 is similar to the Novikov self-consistency hypothesis of time travel, which states that if an action would cause a time travel paradox, its probability is zero. Under this hypothesis, even if entanglement does allow changes to move faster than light--essentially travelling backward in time--any action that would expose this time travel would be forbidden. The scientist is physically prevented from making the "wrong" choice when setting the detector. The problem with this hypothesis is that it seems essentially untestable. Another way to think of option 2 is that the universe is 100% deterministic. Under this hypothesis, the speed of entanglement is only apparent, not real--like planning with a friend to both turn on your flashlights on opposite hilltops 2 hours from now. Without knowing that it was planned in advance, an observer would wonder how you reacted to one another so fast. This is also impossible to fully test; at best you could prove that the consistency extends as far back as we can see into the past. And that is exactly what these scientists will be testing.
- jmcminis 12y ago
  The Novikov self-consistency hypothesis is pretty neat. I like the thought of the researcher travelling back in time to create the apparatus such that the result always confirms quantum entanglement. It's classical and not quantum so it's not clear to me how it relates. I'm not sure anyone (besides philosophers) takes the loopholes seriously.

bhouston 12y ago

Some questions: - Can we use this to send large amounts of data around the world quickly? Can it replace intercontinential data links? - Can we use this for interstellar communication channels of limited amounts of data? - Can we use this to pass messages between two locations in a way that is immune to easedropping? - What are other real-world applications of this?

onewaystreet 12y ago
Unfortunately communication through QE isn't possible: http://en.wikipedia.org/wiki/No-communication_theorem http://en.wikipedia.org/wiki/No-communication_theorem
- Xcelerate 12y ago
  But you can use it to double your bandwidth (http://en.wikipedia.org/wiki/Superdense_coding http://en.wikipedia.org/wiki/Superdense_coding) and "store" bandwidth (http://strilanc.com/quantum/2014/05/03/Storing-Bandwidth-with-Superdense-Coding.html http://strilanc.com/quantum/2014/05/03/Storing-Bandwidth-wit...)
pontifier 12y ago
From my understanding, it cannot be used to transmit information because the measured property of the entangled photon or particle is not chosen by the the experiment. It is truly random. It could be used to ensure that 2 locations have the same random data. This is useful in cryptography.
beloch 12y ago
It's a very common misconception that entanglement allows faster than light communication. It doesn't. Describing quantum phenomena with real world metaphors always breaks down if we carry things far enough. I'm going to use coin flipping as a metaphor to make a specific point, but don't assume this metaphor will still work if you try to extend it. To even begin to understand entanglement you really need to look at the math. That being said, let's proceed. The Setup: (this explains how the metaphor is being made, but you can skip to the payoff if you just want the what without any of the why). We can think of a pair of entangled particles as a pair of coins. If we flip each coin, we have a fifty/fifty odds of gettings heads or tails. This is what we normally think of, in the macroscopic world, as a random outcome (It's not really, but let's not go there). An entangled pair of coins behaves very unusually. The outcome of flipping one coin is related to the outcome of the other coin. For example, if one coin comes up as heads, the other will always come up as tails, or vice-versa. Let's say we separate the two coins by several light years and then flip them. The strange correlation will hold. For a non-Quantum particle there are several possible explanations for this behavior. We might think that, somehow, the measurement of one particle causes the outcome of the other article to change. However, time depends on reference frame. If the two coins are in relative motion to each other in the right manner, which particle was measured first is relative (There is no universally preferred reference frame). i.e. One observer might correctly say that the first coin was measured before the second, but another observer might see things reversed. So, which measurement is the cause and which is the effect? Another notion is that the outcome was preordained when the coins were made. I'd have to go into the math a bit to adequately explain this, but with entangled quantum particles you can create experiments that demonstrate this is impossible. Tests of Bell inequalities are especially notable for this. This is Einstein's "spooky action at a distance", a behavior exhibited by entangled particles. The Payoff: So, if a measurement performed on two entangled particles is like flipping a pair of coins, how can we use that to communicate. Well, we can't. Not directly. You see, we can't force a coin to come up heads or tails. If we look at the outcomes of just one coin they will always appear random and uncontrollable. It's when we talk to the guy with the other coin that we'll say, "Hey, I knew what you were going to get because of what I got!". So, classical communication, limited by the speed of light, is necessary to figure out what these correlations are. So forget about FTL communication. However, what about communicating secrets? Well, that's a whole other kettle of fish! It turns out quantum correlations are fantastic for communicating secret keys. Google quantum cryptography. As for other applications, google quantum computing. Entanglement is absolutely intrinsic to this field. There are also applications in medical imaging and metrology (i.e. standards of measurements).

Xcelerate 12y ago

All components of a quantum system are always entangled. It's not some mysterious property that arises under unusual circumstances. In fact, I would argue that quantum entanglement is the defining feature of quantum mechanics that distinguishes it from classical mechanics. However, the way that most articles (and even physicists) use the term "entanglement" is in a loose way. Within a good approximation, a multi-component quantum state can be represented as a tensor product of single component quantum states. When this isn't a good approximation anymore, you can say that the entanglement of the system is much more apparent. To give an example of what I mean, there's a technique in quantum chemistry known as density functional theory (DFT), which is used to compute ground state energies of various molecules. For some molecules, it works pretty well. The benefit of DFT is that it is a fast calculation technique (well, as far as quantum chemistry goes), but it's speed comes at a price. Rather than using the Coulombic interaction of every pair of electrons to compute the energy of a molecule (or more technically, using every Coulombic interaction as terms in the Hamiltonian), a probability cloud of electron positions is computed instead, and this cloud is used as the energetic term. This works really well for a lot of systems, but in some cases (superconductors, metals, solid-state physics, van der Waals interactions), this approximation falls apart because the effect of electron-electron correlation significantly affects the energy of the system. In this case, the full, non-separable wavefunction is required. The fact that this full wavefunction cannot easily be broken into an approximation of simpler, single electron wavefunctions means that the entanglement of the system is very apparent.

orbifold 12y ago
For someone who is more of a mathematician than physicist, the following article might be of interest in this context: http://math.ucr.edu/home/baez/quantum/node4.html http://math.ucr.edu/home/baez/quantum/node4.html The gist is that the state space of a multicomponent quantum mechanical system (think electrons in a metal) is given by the tensor product of the single component state spaces. As might be familiar from linear algebra, not every element in the tensor product of two vector spaces is simply a tensor product of two vectors. That's really all there is to entanglement, but unfortunately you can't explain that to a lay person.
gus_massa 12y ago

karmakaze 12y ago

I see nothing counter-intuitive about quantum entanglement. Photons travel at the speed of light--time does not exist for them. Alter one, the entangled one is instantaneously connected.

readerrrr 12y ago
That does not work because light speed has a limit. It is true that the photon does not experience time. Imagine a photon traveling a distance of 1 light year towards us( we are stationary). In it's reference frame it will do so immediately but in our it takes 1 year. For the photon to travel to us in zero time in both frames, the speed of light would have to be infinite, which we know it isn't.
krapp 12y ago
"If you think you understand quantum mechanics, you don't understand quantum mechanics." - Richard Feynman
- readerrrr 12y ago
  I wish you comment were downvoted instead of karmakaze's one. He is expressing an opinion and even if he is wrong, it is still an interesting idea, and shows some effort. A decent short explanation could show the mistake, and make a good comment->answer combo. Instead you quoted a comment which is out of context ( Feynman's comment is intended for physicists of his level ), contributing nothing. ( Please don't take this as a personal attack ( it isn't), but more a thought what's wrong with HN in light of recent debate )
  - krapp 12y ago
    No, you're right - it was snide and I should have reconsidered it.
jostylr 12y ago
The issue is with instantaneous. What does that mean in a relativistic context? Depending on the speed of the observer, "instantaneous" changes. This is fine in reconciling some story in the future, but if there is an action happening instantaneously with something else, then we need a notion of same instant. This is easy to add to have something working, but it is extremely hard to add it in a principled way.

danbruc 12y ago

Good opportunity to again promote »The Theoretical Minimum« [1] by Leonard Susskind (born June 1940, director of the Stanford Institute for Theoretical Physics with research interests in string theory, quantum field theory, quantum statistical mechanics and quantum cosmology). It is pretty accessible and a key takeaway is that there is a lot of misinformation in the wild and especially on the internet when it comes to (modern) physics as he tell his class not only once when someone asks about something he read somewhere. [1] http://theoreticalminimum.com/ http://theoreticalminimum.com/

paladin314159 12y ago
This is really cool. I took a (very) basic class on modern physics with Susskind; awesome guy and great at explaining things. Will have to read through this!
- danbruc 12y ago
  Watch, not read, these are video recordings of courses. About 20 courses with about 10 lectures each, usually about 2 hours.
gus_massa 12y ago
The site looks interesting, but it's difficult to find the information that is related to the discussion. Does this site have a page that discusses specifically entanglement? EPR paradox? Bell inecuality? I'd like to see a link to the specific relevant page and not a link to the general site.
- danbruc 12y ago
  Here is the course on quantum entanglement [1] but depending on prior knowledge it may be wise to first go through the courses on classical [2] and quantum [3] mechanics. He tries to keep everything as self-contained as possible but does not go through the very basics things like Lagrangians and Hamiltonians every time. [1] http://theoreticalminimum.com/courses/quantum-entanglement/2006/fall http://theoreticalminimum.com/courses/quantum-entanglement/2... [2] http://theoreticalminimum.com/courses/classical-mechanics/2011/fall http://theoreticalminimum.com/courses/classical-mechanics/20... [3] http://theoreticalminimum.com/courses/quantum-mechanics/2012/winter http://theoreticalminimum.com/courses/quantum-mechanics/2012...

jaekwon 12y ago

I assume that photons act as waves and not particles. Though they have quantized energy (frequency), there's no reason for me to think that they have locality. If so... The most plausible explanation I can think of is a Monty Hall paradox at the source of the emitter. Thousands of photon pairs? How are they generated? Is their emission dependent on the destination via say, pilot wave resonance? Either resonance or waves that travel backwards in time. Experiments of entanglement speed should point towards one or the other.

Ih8registering 12y ago

Good to see the hoax of quantum superstition (superposition) is in its last days. People are wising up!

flatestcat 12y ago
Yeah, there's less rationale for BS like quantumajickalism as grants dry up. That's all this made up BS was designed for --- to scam grant money.