5 ms·
>But (afaik) measuring means disturbing, because you have to exchange some energy with the system in order to perform the measurement. This isn't necessarily t
by gdavisson 8y ago
>But (afaik) measuring means disturbing, because you have to exchange some energy with the system in order to perform the measurement.
This isn't necessarily true; QM allows what's called interaction-free measurement. Say you have a particle that's in a superposition of two states (or places, or whatever), which I'll call A and B. You do something that'll interact (and detect) it if it's in A. If you didn't detect it, that means you've effectively measured it as being in B.
You can also turn it around, and use a particle in a superposition to see if an object (that it would interact with if it's in state A) exists; if the object exists, the possibility of interacting with it will destroy interference effects between the parts of the particle's superposition, allowing you to infer the existence of the object. The Elitzur–Vaidman bomb tester (https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester) is an extreme example of this: it lets you verify that a bomb is "live" (will explode if interacted with), with only a 50% chance of setting it off.
Another example is the interaction-free version of the quantum Zeno effect. The quantum Zeno effect is that (under the right circumstances) a particle can be trapped in a particular state by continuously measuring whether it's still in the state. In the interaction-free version, you use something that'll interact if the particle ever leaves the state... and it never does. This has actually been done; see https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-20150415 https://www.nature.com/articles/ncomms7811?WT.ec_id=NCOMMS-2...
- Moodles 8y agoVery interesting, thanks. This is a nice video to expose the weirdness of the double slit experiment: https://www.youtube.com/watch?v=DfPeprQ7oGc https://www.youtube.com/watch?v=DfPeprQ7oGc
- skipperr 8y agoDoes anyone know of actual videos of the experiment? All the videos I find online only show the wave interference pattern, but not the particle-like pattern shown when measuring/observing the particles.
- spindle 8y agohttp://www.physicscentral.com/experiment/askaphysicist/physics-answer.cfm?uid=20111017091810 http://www.physicscentral.com/experiment/askaphysicist/physi... (I refer you to the photos there, which I think are what you want. I haven't read the text.)
- deleted 8y ago[deleted]
- Koshkin 8y ago> you've effectively measured it as being in B Doesn't sound right to me. Being in a superposition of A and B doesn't necessarily mean that the system is in either of the two states.
- _zachs 8y agoIf you're measuring a bit and it's not 0, what is the only other value it can be?
- Koshkin 8y agoIn the case of an "interaction-free" measurement (i.e. one that supposedly would not disturb the system), as mentioned by the parent, if the interaction did not happen, then the system continues to be in a superposition of the two base states (0 and 1, in your example).
- amluto 8y agoCitation needed. In a two-level quantum system, if I make any measurement of a particle such that the distribution of outcomes depends on whether the particle is in state 0 or 1, then I have disturbed the state of the system. This is true no matter how fancy the measurement is. This can’t even be avoided with hacks that depend on detailed knowledge of the system. If the initial state is |0>+|1>, then, sure, I can measure and then reset the state. But if the initial state was entangled with some far-away particle, I can’t reset the state of the nearby particle without completely forgetting the outcome of the measurement.
- Koshkin 8y agoYeah, this "interaction-free" measurement didn't sit well with me, either.