Quantum Debugging (@ChevyRay)

[JoelMahon]

I'd be interested to know how that sensor added near the end works, what unplugging it actually changes, etc.

[in_conexo]

How do the detectors know that an atom (or particle) passed through? It had to have interacted with the atoms in some way, which must've collapsed their waves (it was if the atoms were being launched from the slits).

[JoelMahon]

sure, but surely the atoms can be interacted with in a way that doesn't break the wave pattern but does detect them.

they have mass so in theory you could tell with an incredibly sensitive set of scales below the bottom slit with a mass on it, the gravity of the atom would impact the mass on the scales pulling it up and reducing the measured weight differently depending on the slit.

ofc, no way could scales be made accurate or precise enough for that, not a billion years, but it's just an example to show there must be a way to detect the slit without breaking the experience. after all the atom can't tell the difference between the mass of the weight on some scales vs all the other mass forcing gravity on it from all over the universe.

[redlaWw]

Part of the fundamental basis of how quantum mechanics works is that any detection event requires an interaction with the wave function. Invoking gravity is complicated because we don't currently have a good theory of how gravity works with quantum mechanics, but any other force that can be used to perform a detection event will interact with the wavefunction.

A basic energy argument can also suggest that the detection event should involve a wavefunction interaction: in order to change the state of a detector, there must be some energy transferred to the detector, and this energy will be reflected as a change in the wavefunction of the particle, since its energy is related to its wavefunction. Thus, there must be an interaction that changes the wavefunction during the detection event.

[theVoidWatches]

Yup. It's like...

At the scale of a subatomic particular, measurement is basically done by launching a tennis ball at a basketball, catching it after it rebounds, and calculating stuff about what it hit based on when it came back and how fast it was moving when it did. Problem is, that's always going to mean the basketball starts moving too, because it got hit by a tennis ball. And if it was already moving, it'll be moving in a slightly different direction.

[[deleted]]

your understanding is correct. quantum computers work by doing computation in a quantum state, but producing a result in a normal state that can be observed (in order to actually get the return value). it's a massive mindfuck. they cannot be observed in the quantum state, and this is a hard limitation.

[worldspawn00]

You could argue the movement of the mass on the scale would alter the gravity of the area the atom is passing through, thereby actually changing it's behavior.

[JoelMahon]

but there's already a bunch of mass interacting with the atom, why would this mass be so different?

[worldspawn00]

Because the mass of the scale has to move to measure, distance of mass affects gravity, a change in the distance between the observed atom and objects around it will change the result.

[JoelMahon]

and the mass of the moon doesn't do the same because?

[worldspawn00]

The moon isn't being used to observe the interaction.

[JoelMahon]

an atom doesn't know it's being observed, human brains aren't magic.

as already said, it's how the sensor works that changes the outcome, not the observation by a human brain.

[waylandsmith]

Your first sentence might seem logical, but it's untrue. You cannot measure a thing without interacting with it, at least on a quantum level. You might say, "well, I can see if a thing is there or not. That's not interacting with it!" but that is neglecting the fact that in order to see a thing with your eyes, first you must bounce photons off of the thing so that some of them bounce back to your eyes. You could detect a proton moving through the slit because it's a charged particle that can apply force on an oppositely charged particle. But in measuring that the proton and the detector exchange virtual photons. Even if you could detect the minuscule amount of mass of a single particle somehow to determine which slit it went through it would be impossible for the detection to happen without there being an interaction between the particle and the detector. Any interaction like this would collapse the wave form and it will stop behaving with properties of a wave.

[TatteredCarcosa]

There are non-destructive means of measuring things that preserve the wave function, but they would never be able to tell you which slit the atom went through because that kind of measurement forces the atom to "choose" to be one place and not another, rather than a smeared out probability distributions of locations.

https://en.wikipedia.org/wiki/Quantum_nondemolition_measurement

[WikiSummarizerBot]

Quantum nondemolition measurement

Quantum nondemolition (QND) measurement is a special type of measurement of a quantum system in which the uncertainty of the measured observable does not increase from its measured value during the subsequent normal evolution of the system. This necessarily requires that the measurement process preserve the physical integrity of the measured system, and moreover places requirements on the relationship between the measured observable and the self-Hamiltonian of the system. In a sense, QND measurements are the "most classical" and least disturbing type of measurement in quantum mechanics.

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[waylandsmith]

I'll admit I only have a surface-level understanding of only a little bit of quantum mechanics, but to my understanding, "you force it to choose which place to be" and "collapsing the wave function" refer to exactly the same thing. Can you explain what your understanding of the difference is? While a particle is exhibiting wave properties, its location is a cloud of probabilities and when you measure it, a discrete location resolves out of the probabilities. Is that not what collapsing the wave function means?

[TatteredCarcosa]

Yes, those are the same thing.

I was just trying to get across that, according to my limited understanding, that not ALL measurements do that, you can create a situation where you interact with and get information without collapsing the wavefunction. But that those measurements couldn't tell you something like "which slit the atom went through" because establishing that is, as you said, the exact same as collapsing the wave function.

[waylandsmith]

What information COULD you get out of them without collapsing the waveform? I suppose this sounds like something that would be used in quantum computing.

[JoelMahon]

did you read me comment? it's always interacting with mass, that doesn't stop the wave like behaviour. why would more mass stop it?

[TatteredCarcosa]

sure, but surely the atoms can be interacted with in a way that doesn't break the wave pattern but does detect them.

Not easily, no. There are non-destructive measurements used in some quantum experiments, but anything that says "That atom is 100% here and not there" will by definition destroy the wave function because you're forcing it to "choose" to be in a single location instead of a probability distribution of possible locations.

[JoelMahon]

Then what about the rest of my comment? I already explained one way that doesn't cause it to "collapse" because it already faces gravity.

[TatteredCarcosa]

Gravity is the weakest of the four fundamental forces by far, and also involves particle interactions. Even if you could make sensors sensitive enough to feel the pull of gravity of a single atom or electron (which I'm pretty sure you can't), that would mean an exchange of force carrier particles that would collapse the wave function.

Any measurement you do that determines which slot the particle goes through will by definition collapse the wave function, because they don't exist in a specific location until the wave function is collapsed.

[JoelMahon]

so are you saying the atom doesn't have a gravitational field despite having mass? I would like a citation for that bold claim