5 ms·

An Introduction to Quantum Computation and Communication by Rob Pike [PDF]

28 points by caustic 15y ago

elemenohpee 15y ago
I'm about as far from an expert as you can get, so hopefully someone more knowledgeable can shed some light on this: "A classical computer seems to need time exponential in n to predict precisely the behavior of a general quantum mechanical system of n particles. (Yet nature manages to do it in real time.)" How could we know this from inside that system? For all we know it could take a billion [whatever unit is used to measure time outside of our dimension] to calculate each step, and it would still look fluid to us.
- brg 15y ago
  The usual time measure of computational complexity is in units of computation, not in terms of the wall clock time it takes to for a calculation to run.
  - elemenohpee 15y ago
    Then what does "real time" mean in this context?
- palish 15y ago
  Because, read Feynman's paper. No really, read it. His writing style is very accessible. Ignore the formulas; read the words. http://www.phy.mtu.edu/~sgowtham/PH4390/Week_02/IJTP_v21_p467_y1982.pdf http://www.phy.mtu.edu/~sgowtham/PH4390/Week_02/IJTP_v21_p46...
scythe 15y ago
http://scottaaronson.com/blog/?p=208 http://scottaaronson.com/blog/?p=208 -- possibly helpful for those who saw the Shor's algorithm bit and wanted more.
palish 15y ago
The two-slit experiment. 1. Single photon still produces interference pattern! 2. Ask which slit photon passes - pattern disappears This kind of explanation is common, and it has always bothered me, because it's the wrong sort of mental model. The interference pattern doesn't disappear "because we're looking", or "because we're asking", or "because we thought about finding the answer". It disappears because in order to check which slit a photon passed through, we need some way of measuring that. To do so, we need some way to "see" the photon; to do that, we need to shine photons on it; and it is that which destroys the interference pattern. The reason quantum mechanics is "weird" is because it (currently) is fundamentally impossible to invent a device to answer the question "which slit did the photon pass through?" without destroying the interference pattern. However, that doesn't mean it disappears "because we're asking". The interference pattern is destroyed because our device, no matter what it is, will always interfere with the experiment (shining photon A at a photon B == "well obviously that would change the behavior of photon B"). Nothing more, nothing less.
- tzs 15y ago
  A better example of "weird" than the two slit experiment is the the quantum bomb detector, which basically answers the question "if I sent a photon through point X, would it be absorbed?" without actually sending a photon (or anything else measurable) through point X. There's no attempt with this to determine where photons are going, so you don't have the pesky issues of observation attempts messing with your photons. http://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb-tester http://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb-tester Here's an improved version that doesn't explode as often: http://njsas.org/projects/light_polarization/Interaction-Free-Measurements.htm http://njsas.org/projects/light_polarization/Interaction-Fre...
  - deleted 15y ago
    [deleted]
  - foob 15y ago
    There's no attempt with this to determine where photons are going, so you don't have the pesky issues of observation attempts messing with your photons. That's not true, this is no different than the double-slit experiment in that respect. If you have a dud bomb (one with no photon detector) then the wave function is propagating along two paths simultaneously and you get interference. If you have a real bomb (one with a photon detector) then it interacts with the wave function in such a way that the photon can only take one of the two paths and you no longer get interference. The details of the setup are slightly different than with the double-slit but the physics and explanation are essentially identical.

28 points by caustic 15y ago

I'm about as far from an expert as you can get, so hopefully someone more knowledgeable can shed some light on this: "A classical computer seems to need time exponential in n to predict precisely the behavior of a general quantum mechanical system of n particles. (Yet nature manages to do it in real time.)" How could we know this from inside that system? For all we know it could take a billion [whatever unit is used to measure time outside of our dimension] to calculate each step, and it would still look fluid to us.

brg 15y ago
The usual time measure of computational complexity is in units of computation, not in terms of the wall clock time it takes to for a calculation to run.
- elemenohpee 15y ago
  Then what does "real time" mean in this context?
palish 15y ago
Because, read Feynman's paper. No really, read it. His writing style is very accessible. Ignore the formulas; read the words. http://www.phy.mtu.edu/~sgowtham/PH4390/Week_02/IJTP_v21_p467_y1982.pdf http://www.phy.mtu.edu/~sgowtham/PH4390/Week_02/IJTP_v21_p46...

scythe 15y ago

http://scottaaronson.com/blog/?p=208 http://scottaaronson.com/blog/?p=208 -- possibly helpful for those who saw the Shor's algorithm bit and wanted more.

palish 15y ago

The two-slit experiment. 1. Single photon still produces interference pattern! 2. Ask which slit photon passes - pattern disappears This kind of explanation is common, and it has always bothered me, because it's the wrong sort of mental model. The interference pattern doesn't disappear "because we're looking", or "because we're asking", or "because we thought about finding the answer". It disappears because in order to check which slit a photon passed through, we need some way of measuring that. To do so, we need some way to "see" the photon; to do that, we need to shine photons on it; and it is that which destroys the interference pattern. The reason quantum mechanics is "weird" is because it (currently) is fundamentally impossible to invent a device to answer the question "which slit did the photon pass through?" without destroying the interference pattern. However, that doesn't mean it disappears "because we're asking". The interference pattern is destroyed because our device, no matter what it is, will always interfere with the experiment (shining photon A at a photon B == "well obviously that would change the behavior of photon B"). Nothing more, nothing less.

tzs 15y ago
A better example of "weird" than the two slit experiment is the the quantum bomb detector, which basically answers the question "if I sent a photon through point X, would it be absorbed?" without actually sending a photon (or anything else measurable) through point X. There's no attempt with this to determine where photons are going, so you don't have the pesky issues of observation attempts messing with your photons. http://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb-tester http://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb-tester Here's an improved version that doesn't explode as often: http://njsas.org/projects/light_polarization/Interaction-Free-Measurements.htm http://njsas.org/projects/light_polarization/Interaction-Fre...
- deleted 15y ago
  [deleted]
- foob 15y ago
  There's no attempt with this to determine where photons are going, so you don't have the pesky issues of observation attempts messing with your photons. That's not true, this is no different than the double-slit experiment in that respect. If you have a dud bomb (one with no photon detector) then the wave function is propagating along two paths simultaneously and you get interference. If you have a real bomb (one with a photon detector) then it interacts with the wave function in such a way that the photon can only take one of the two paths and you no longer get interference. The details of the setup are slightly different than with the double-slit but the physics and explanation are essentially identical.