
Elitzur–Vaidman bomb tester - shawndumas
https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester
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QAPereo
This one of the great (originally thought) experiments, but it comes with a
serious fundamental caveat...

 _...although there is a 50% chance that the bomb will explode in the effort._

Of course you could imagine using this device and a massive thermonuclear bomb
on your lap to test notions of so-called Quantum Immortality. Please note that
YMMV when attempting to play that game.

~~~
Strilanc
You can tweak the experiment to get arbitrarily close to 0% chance of
exploding the bomb.

Instead of using a 50/50 beam splitter and passing the photon through the
system once, you pass the photon many times while gradually rotating it from
the free path onto the bomb path. If the bomb isn't present, you end up with a
photon on the empty bomb path. If the bomb is present, the zeno effect will
keep the photon on the free path. Then you just check which path it ended up
on in the end.

~~~
saagarjha
Yes, but doesn't the repeated application increase the probability again? How
is this bringing it closer to 0%?

~~~
Strilanc
For a rotation angle of x° the probability of failure for each check+rotate
when a bomb is present is sin(x°)^2. It takes 90°/x° repetitions of a rotation
by x° to get a useful effect in the safe case.

So the overall failure rate is 1-(1 - sin(x°)^2)^(90°/x°) or equivalently
1-cos(x°)^(180°/x°).

Compute the limit as x goes to 0 and you'll find the limit equals 1. Making x
smaller increases the chance of success arbitrarily close to unity.

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jonbaer
Neutrinos might be a better bet ...
[https://motherboard.vice.com/en_us/article/9akwb7/how-
neutri...](https://motherboard.vice.com/en_us/article/9akwb7/how-neutrinos-
can-help-track-nuclear-bombs)

~~~
dogma1138
These 2 things are utterly unrelated.

One is a physical principle of interaction free measurement the other is just
measuring neutrinos.

