

An Introduction to Quantum Computation and Communication by Rob Pike [PDF] - caustic
http://herpolhode.com/rob/qcintro.pdf

======
palish

      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
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>

Here's an improved version that doesn't explode as often:

[http://njsas.org/projects/light_polarization/Interaction-
Fre...](http://njsas.org/projects/light_polarization/Interaction-Free-
Measurements.htm)

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elemenohpee
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
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
Then what does "real time" mean in this context?

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

