Talk:Elitzur-Vaidman bomb-tester

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Aren't the mirrors incorrectly orientated?

Yes, the one is, top left corner. -Brak710101

No, all four are. Plus there was no distinction made between half-silvered plane mirrors and fully silvered plane mirrors. Also, the bomb detector needs to lie on the lower path. I've uploaded a correction. Thoreaulylazy 17:17, 23 February 2006 (UTC)

By 'device' you mean the upper right hand half-silvered mirror? also when listing the scenario explaining which interferon (C or D) recieves or doew not recieve a photon might help. Stupid people like me need half-steps. I'm guessing in scenario (1) no photon is observed, in (2) D observes photon, (3) D observes photon, and (4) C observes photon? HiS oWn 18:53, 23 February 2006 (UTC)

I hope it's clearer now. I created nested If branches to explain what all happens under each possibility. Thoreaulylazy 20:06, 25 February 2006 (UTC)

  • Sexy informative. HiS oWn 00:27, 7 March 2006 (UTC)

[edit] superposition

"... even though only one path is actually taken" That is a classical point of view and has no real meaning in quantum mechanics. The usual interpretation is that the photon does go through all possible paths, not that it somehow remotely "senses" them. Moo 02:00, 12 February 2007 (UTC)

Not when an observer is introduced. An observer, at the moment of observation, collapses the wave function, so the photon cannot be at two places at once. The bomb detector acts as the observer. —The preceding unsigned comment was added by Thoreaulylazy (talk • contribs) 21:44, 3 March 2007 (UTC).

[edit] Kwiat Reference?

In 1996, Kwiat et al. have devised a method, using a sequence of polarising devices, that efficiently increases the yield rate to a level arbitrarily close to one.

First of all, polarizing is spelled incorrectly. Secondly, can anyone come up with a reference to this method? I'd love to read more about it. --216.152.208.1 03:40, 16 July 2007 (UTC)

[edit] "Thought experiment"?

Given that it's actually been performed (albeit not with actual bombs) is it still fair to call it a thought experiment? Schrödinger's Cat was an untestable thought experiment designed to seem impossible and thus cast doubt on Quantum Mechanics as a whole. This experiment, on the other hand, was designed to be testable and was successfully tested within a year of its proposal--why, then, is the article titled "Elitzur-Vaidman bomb-testing problem" if we've already received a definitive answer that appears to agree with Copenhagen QM? Why is it referred to in the first sentence as a thought experiment, and only after reading two thirds of the article am I informed that it has actually been successfully tested?

I think that the successful physical experiment is a rather large point of interest--one that shouldn't be mentioned as a mere aside towards the end of the article. --Lode Runner 03:13, 3 October 2007 (UTC)

You are right--- why don't you put a statement about the experiment somewhere early? By the way, I think the reason it's still called a thought experiment is because there is reasonable consensus on what Quantum Mechanics predicts. So when someone finds a surprising prediction, people expect it to turn out like QM says, even if there is no experiment yet. The bomb testing problem is most significant not because it is experimentally verified ( everybody expected that ) but because it clarifies an important property of quantum mechanics explored in depth by later articles by Vaidman, namely that quantum mechanics allows you to experimentally test philosophical counterfactuals. This is a striking fact, which suggests a verifiable sense in which quantum mechanics is a many-worlds theory.Likebox 03:36, 3 October 2007 (UTC)
Or, to put it another way, it's remarkable in that it produced a measurable macroscopic effect that perfectly corresponds with QM's predictions (as you say, the many-worlds interpretation specifically--how else can you comprehend it, if not to say that in some "alternate universe" the bomb did in fact go off and prevent the photon from advancing any further?) This is what Schrödinger's Cat was originally designed for... but I thought that the irony of the cat was it could never be actually physically tested. It never occurred to me that someone would come up with an actual testable version of the problem. I'm not a physicist (quantum or otherwise), but I enjoy learning about it in my spare time and... I must say, I'm pretty damned shocked that we've achieved definitive experimental proof of a macroscopic "counterfactual", as you say. This is quite possibly the most astounding thing I've read in the past ten years, (and the sad part is the experiment itself is 13 years old... completed one year before I first checked out a book on Quantum Mechanics from my middle school library.)
Will do the edits tomorrow, after my head has stopped spinning quite so much. --Lode Runner 03:59, 3 October 2007 (UTC)
I had the same reaction when I heard about it. It's astonishing that you can measure a counterfactual. I was an undergrad around when the paper came out. Or rather, I should say, it didn't come out exactly as spread by word of mouth. It was supposed to be published in a big-name journal, but a big-name referee didn't feel confident to judge it's quality (I heard it from him firsthand), and another referee rejected it because "it's just quantum mechanics. Nothing new here." ! So this amazing result got published in a ridiculously obscure journal, and from there spread from graduate student to graduate student by word of mouth. We had a good time figuring out how to get close to 100% efficiency.Likebox 05:28, 3 October 2007 (UTC)
I took the liberty of renaming the article to reflect the experiment's status as a working apparatus (instead of an unsolvable or infeasibly-tested thought experiment "problem"), mentioned the successful test early on and added the extra category of "physics experiment." It deserves a better writeup than this--something on par with the Double-slit experiment or even the Michelson–Morley experiment, IMO, but that's a task for someone better qualified than myself. --Lode Runner 08:38, 13 October 2007 (UTC)
Just a comment: Please, if you have time, read about QM, learn what you can, and write when you feel you understand it well. If you screw something up, somebody will correct you. The sad fact of this world is that we're all stupid. If you wait for someone smart to come along, all you get is some charlatan who is pretending not to be stupid. Personally, I don't think I can give it the write up it deserves, because it's been so long since I really felt the revolutionary power of the result. I also had some ideas about it which I wouldn't be able to stop myself from mentioning and that would definitely be OR. Either Elitzur or Vaidman would probably go on and on about all the insights they've had since then, which are really great, but probably not as useful for a newcomer. So maybe you are the most qualified.Likebox 15:39, 15 October 2007 (UTC)
To me, the experiment proves the MWI of quantum mechanics. The simplest explanation is that all the bombs do actually explode - but in different future worlds. In one world around 50% of the (live) bombs explode, and 50% do not. Keeping a live bomb and knowing it is live is the equivalent of taking a peek into the box with the cat when mother nature isn't looking. It's absurd, in my mind, to imagine that the bomb was sampled without being sampled - it was sampled in another universe, it exploded in that universe, and occasionally we get to see the result of that explosion in our own universe with this elegant trick. This is how quantum computers work, and it shows that MWI is obviously true, no matter how mind-boggling that may be. Can some extension of the experiment be performed? Say, labeling all the bombs first, seeing which bomb explodes then using that information to choose which bomb is tested next? Could this allow some sort of communication between universes? --, 3 January 2008 —Preceding unsigned comment added by 76.184.217.42 (talk) 14:16, 3 January 2008 (UTC)
Quick comment--- the experiment does show that there is a cerrain many-worlds aspect to our universe, because the fact that the bomb-sensor would measure the photon can affect a branch where it doesn't happen. The wavefunction is many-worlds enough to "sniff out" that the device would measure the photon were it to go there.
But this experiment does not conclusively show that the bomb actually went on to explode in this branch. Perhaps, right after the initial measurement that branch of the wavefunction deletes itself out of existence. Using the formalism of ordinary quantum mechanics this is not possible--- there is no wavefunction reduction. But perhaps QM is not completely right. It is not conclusively established by the experiment that all stages of the measurement occur, just that the first few stages. Once the measurement cascades far enough so that a bomb actually goes off, that branch is forever sealed off from direct observation or communication. I think a careful statement is that this experiment implies, in reasonable practical terms, either Everett/many-histories/something equivalent, or a testable modification of quantum mechanics which includes a collapse mechanism.Likebox (talk) 21:57, 4 January 2008 (UTC)