

A switch that lets one photon alter the quantum state of another - scottshea
http://www.physorg.com/news/2011-09-photon-quantum-state.html

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IvoDankolov

      And if the first photon was in some weird quantum state, where it can’t be said to have struck the atoms or not, the second photon will be both extra-delayed and not extra-delayed at the same time.
    

Maybe it's just me, but these always come out sounding massively hand-wavy and
scare off a lot of people (or make you look like an idiot, depending on how
much respect they have for physicists). I think the fix would be mostly a
matter of coming up with terminology that doesn't fall flat on its ass, but
still remains accessible, because, on the other hand, speaking of amplitude
distributions would kind of alienate a lot of people.

Anyway, much more importantly - it's a clever little engineering trick to
entangle the two photons. However, and I will say I have not researched the
topic of quantum computing - doesn't operating them require interference
between the different branches of the system at some point to get meaningful
results? If so, you would need to get two or more branches to end up in
exactly the same state, and nothing in the article mentions that the cesium
atoms that have been altered will reset without outside influence.

Anyway, I put the conditional probability that I know everything about the
system or quantum mechanics awfully close to 0, so it's more of a curiosity
(how would you make it work?) instead of a genuine concern.

~~~
sp332
That terminology is an approximation of explaining the math that is used to
describe what's going on at a quantum level. Basically you end up with a
matrix that has one part describing one outcome, and another part describing a
different outcome. Both outcomes have to be considered part of the same
system, you can't have one without the other. For example, the "quantum bomb
tester"
[https://secure.wikimedia.org/wikipedia/en/wiki/Elitzur%E2%80...](https://secure.wikimedia.org/wikipedia/en/wiki/Elitzur%E2%80%93Vaidman_bomb_tester)
requires both paths to be available at the same time, or the system would
behave differently. This isn't just a thought experiment, it's been
experimentally verified. Without the bomb of course :)

~~~
IvoDankolov
Oh, don't get me wrong, I have no trouble with multiple outcomes, or to put it
in a different way, the whole wavefunction being real. Otherwise you'd have a
pretty strange time explaining interference ("oh, one thing is real, but it
considers the other possibilities anyways" or "oh no, the waveform hasn't
collapsed just yet, but it will once we observe it"). In fact, the Feynman
path integral, i.e. the sum over all histories, appeals a lot to me on a
conceptual level.

But that isn't the point really. I just feel like saying "oh, it's in some
weird quantum state of being both this and that at the same time" is a bit
disingenuous, since it is not true in any classical sense, which, shockingly,
most people would assume and get utterly confused. It is necessarily true if
you consider many worlds as the valid interpretation (which I do like, but
let's not go there), but it doesn't I think hint that well enough, nor does it
hint that we have entered a cloud of possibilities that is about to collapse
if that was the point.

More than just being clear on what is physically happening (and I do say "oh,
it's just equations, they have no meaning" is the biggest hand waving of all),
it feels kind of immature. The tone hints to a confusion in the speaker, as if
we've just gotten these strange experimental results and we're not sure how to
deal with them yet. It's something that could've been acceptable when
practically nothing was known about quantum mechanics, but more than half a
century later?

And to be perfectly fair, I'm not sure how you would phrase the wavefunction /
amplitude distribution to at least give someone without knowledge of the
subject a workable model. Maybe the inferential distance is too long to even
have a not completely wrong intuition, but I feel we can do better.

