Quantum Mechanics and Quantum Computation Course

https://www.edx.org/course/quantum-mechanics-quantum-computation-uc-berkeleyx-cs-191x

by austinlyons • 12 years ago

3 5 12 years ago

jndsn4023• 12 years ago

I have a math background, no physics. Whenever I try educating myself about this stuff I get hung up on the contradictions and can't/don't want to just take certain concepts as given in order to understand more advanced concepts.

For example (from the pdf of the intro to this course, https://courses.edx.org/c4x/BerkeleyX/CS-191x/asset/chap1.pd...):

"Logically, we can ask which slit the photon went through, and try to measure it. Thus, we might construct a double slit experiment where we put a photodetector at each slit, so that each time a photon comes through the experiment we see which slit it went through and where it hits on the screen. But when such an experiment is performed, the interference pattern gets completely washed out! The very fact that we know which slit the photon goes through makes the interference pattern go away."

I read that and say, really? The fact that we know about it? How does the photon know that we know about it? I always feel like this is just a layman's way of describing what happens, but there is actually a more rigorous understanding among physicists. Is that the case? Because after reading that sentence I want to stop right there and redo that experiment until we figure out what's really going on.

tzs2• 12 years ago

How does the photon know that we know about it?

The next sentence after the part you quoted is the key: "This is the first example we see of how measuring a quantum system alters the system".

To measure the system in order to know which slit it went through, we had to physically interact with it, and that changes its behavior. The experiment is no longer "send photons through these double slits and then count them over there". It's "send photons through these double slits, make them interact with our measuring doohickey right behind the slits, and then count them over there".

Take a look at Feynman's coverage of this in Chapter 1 of Volume III of "The Feynman Lectures on Physics". You can find them online in a beautiful HTML version here: http://www.feynmanlectures.caltech.edu

jndsn4023• 12 years ago

I will check out the Feynman lectures, thanks for the reference. But my initial reaction is, isn't the measuring thing somewhere past the slit? So how could its presence there influence which slit the photon goes through?

Quantum Mechanics and Quantum Computation Course

Related Stories