4 ms·

Elitzur–Vaidman bomb tester

55 points by shawndumas 9y ago

QAPereo 9y ago
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 9y ago
  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 9y ago
    Yes, but doesn't the repeated application increase the probability again? How is this bringing it closer to 0%?
    - Strilanc 9y ago
      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.
tritium 9y ago
So, the only practical use for this test would be a russian roulette game show in which contestants compete to try their hand at discovering which of the four boxes contains a bomb. Pass. I’ll just play minesweeper.
jonbaer 9y ago
Neutrinos might be a better bet ... https://motherboard.vice.com/en_us/article/9akwb7/how-neutrinos-can-help-track-nuclear-bombs https://motherboard.vice.com/en_us/article/9akwb7/how-neutri...
- dogma1138 9y ago
  These 2 things are utterly unrelated. One is a physical principle of interaction free measurement the other is just measuring neutrinos.

55 points by shawndumas 9y ago

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 9y ago
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 9y ago
  Yes, but doesn't the repeated application increase the probability again? How is this bringing it closer to 0%?
  - Strilanc 9y ago
    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.

tritium 9y ago

So, the only practical use for this test would be a russian roulette game show in which contestants compete to try their hand at discovering which of the four boxes contains a bomb. Pass. I’ll just play minesweeper.

jonbaer 9y ago

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

dogma1138 9y ago
These 2 things are utterly unrelated. One is a physical principle of interaction free measurement the other is just measuring neutrinos.