recently i was watching a video on quantum computing where the narrators describes that quantum entanglement information travels faster than light!

Is it really possible for anything to move faster than light? Or are the narrators just wrong?


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    $\begingroup$ I must unfortunately state that, at the present day, anything you read or hear in the popular media about quantum computing should be treated with deep suspicion. (I say this as someone who works in the field!) The problem is the media is absolutely full of total garbage about the subject, in part because of the existing culture surrounding popular presentations of QM (which is also largely garbage, with a few notable exceptions: e.g. Penrose, Hawking, and other such luminaries). If something said about QC sounds fantastic, then you should expect that it is close to being totally false! $\endgroup$ – Niel de Beaudrap Oct 1 '11 at 18:33
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    $\begingroup$ (I would like to add: models of quantum computing do have intriguing properties which surpass anything we know how to do with classical computers, and it's realistic to hope that we build them some day. However, they are not magical, nor paradoxical. Their properties are just bold extensions of the properties of classical computers, when you add one or two extra ingredients. Entanglement, for instance, is an exotic sort of correlation; but that's all that it is --- correlation of random results --- albeit one of a peculiar sort, which one could not even describe in "classical" probability.) $\endgroup$ – Niel de Beaudrap Oct 1 '11 at 18:39
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    $\begingroup$ @Niel: The problem with describing entanglement as probability correlation (although it is the direct quantum analog) is that correlation can be always interpreted as ignorance of hidden variables, while quantum entanglement has no local ignorance interpretation. $\endgroup$ – Ron Maimon Dec 13 '11 at 14:06
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    $\begingroup$ @Ron: I am not describing it as being a merely classical correlation, though. If we define "correlated" as just being "not independent", the fact that entanglement is a form of correlation immediately follows. The fact that there is no intuitive ignorance interpretation doesn't really affect this. $\endgroup$ – Niel de Beaudrap Dec 13 '11 at 20:08
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Collapsing an entangled pair occurs instantaneously but can never be used to transmit information faster than light. If you have an entangled pair of particles, A and B, making a measurement on some entangled property of A will give you a random result and B will have the complementary result. The key point is that you have no control over the state of A, and once you make a measurement you lose entanglement. You can infer the state of B anywhere in the universe by noting that it must be complementary to A.

The no-cloning theorem stops you from employing any sneaky tricks like making a bunch of copies of B and checking if they all have the same state or a mix of states, which would otherwise allow you to send information faster than light by choosing to collapse the entangled state or not.

On a personal note, it irks me when works of sci-fi invoke quantum entanglement for superluminal communication (incorrectly) and then ignore the potential consequences of implied causality violation...

  • $\begingroup$ so, it means whatever you do with quantum entanglement, the information flow is always bound to $c$. Change of state of spin(for example) does travel at speed of light to affect at the other end. $\endgroup$ – Vineet Menon Oct 2 '11 at 5:18
  • $\begingroup$ You cannot change the spin on one particle and get a corresponding change on the second particle. Niel de Beaudrap sums it up very well as a 'correlation of random results'. Once you make a measurement (i.e. interact meaningfully with one of the entangled particles) the entanglement is collapsed. $\endgroup$ – Richard Terrett Oct 2 '11 at 5:42
  • $\begingroup$ so, is it that one cannot use entanglement for any use? since a single measurement destroys the entanglement itself! $\endgroup$ – Vineet Menon Oct 2 '11 at 6:50
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    $\begingroup$ Sure, you can use entanglement. You can use it for the same things that you can use correlated random results for: for example, you can use it to turn insecure public communication between distant parties, into secure private communication. And a number of other intriguing theoretical applications. Just not for instantaneous communication, or anything similar to it. $\endgroup$ – Niel de Beaudrap Oct 2 '11 at 9:24
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    $\begingroup$ @KonradRudolph - Both appear to be correct: the no-cloning theorem forbids superluminal communication through cloning of states. However this is 'sufficient but not necessary', as the article states, as the theorem does not say anything about other possible techniques that don't employ cloning of states. Perhaps I should have said 'any sneaky tricks (that employ cloning of states)' to be more clear. $\endgroup$ – Richard Terrett Feb 27 '12 at 3:05

There are many things that happen faster than speed of light. For example when big bang happened at the beginning of universe, the expansion of Universe is faster than speed of light. If you have studied Bell's theorem, it states and proved by experimentation that nature itself is fundamentally nonlocal. Nonlocality is in the form of instantaneous collapse of wave function. Another example is, if a bug flies across the beam of a movie projector , the speed of its shadow is proportional to the distance to the screen: in principle that distance can be as large as you like and hence the shadow can move arbitrarily at a high velocity. Note: The shadow of the bug moves across the screen at a velocity greater than c, provided the screen is far far enough away. Its true. However the shadow does not carry any energy or transmit any message. Another example is ethereal influences in EPR Experiment. Likewise there are many examples but the important point nothing carries energy or a message from point A to point B.

  • $\begingroup$ I don't think this really clarifies anything. I appreciate that you are saying that nothing "physical" is transmitted at light-speed or faster (which is true). But you undermine yourself by talking about "ethereal influences" in the EPR experiment. Which is it --- is there an actual 'influence, or not? $\endgroup$ – Niel de Beaudrap Oct 1 '11 at 18:48
  • $\begingroup$ There are ethereal influences. That's how the conservation of angular momentum is done. Yes there is an actual influence. $\endgroup$ – deepthought Oct 1 '11 at 19:36
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    $\begingroup$ The usual interpretation of Bell's inequality violations is that the universe is non-real (i.e. no hidden variables), not non-local. $\endgroup$ – user2963 Oct 1 '11 at 20:49

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