
An Animation About Quantum Computers - jonbaer
http://quantumfrontiers.com/2013/08/22/the-most-awesome-animation-about-quantum-computers-you-will-ever-see/#content
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kghose
I'm sorry, but this video did not make anything clear to me. I really liked
the visuals but there was absolutely no point in the video that I could find.
I feel the narrators could have done a better job explaining what quantum
computation IS, rather than going on about superposition. Are these parallel
computations going on? What?

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ssivark
Unfortunately, that video makes a lot more sense only if you understand
quantum mechanics or quantum computation.

The video doesn't realy describe _what quantum computation is_ but it rather
describes _what feature of QM makes quantum computers different from classical
computers_ (superposition and quantum correlations) and _why is it so
difficult to build or use a quantum computer_ (decoherence, and the fact that
you can't observe or clone the "registers" at intermediate stages in a
computation).

The video addresses the notion of quantum computing quite obliquely. I would
expect this to be the first of a multi-part series where this is explored in
more depth (especially given the attention it's receiving).

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sophacles
It is - well its part 2 of an N-part series. Apparently they are being
released as they are being finished, the last one having come about about a
month ago. The link is in the text a handful of pixels below the youtube
embedding.

~~~
ssivark
Ah, okay. While skimming, I mistook that to mean that this video on quantum
computing was the second in their series exploring diverse topics.

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fatjokes
Jorge Cham is my hero. What's the most logical step after getting a mechanical
engineering PhD from Stanford and a post-doc at Caltech? Become a cartoonist,
of course.

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ibudiallo
Finally after watching this video, I can tell with great confidence I don't
understand the first thing about quantum computing.

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FrankenPC
Anyone know the answer to this question? OK...so, if multiple wave functions
which are overlapped make up the concept of super-position, and we can measure
the wave function at any given time, why can't we take enough measurements to
formulate a wave theory that will predict where the wave will be at ANY given
time?

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Xcelerate
> we can measure the wave function at any given time

We can't. Nobody knows how to measure a wavefunction. Instead, what you
measure is one of the eigenvalues of a linear operator that acts on a
wavefunction (which particular eigenvalue? You can't always predict this;
hence the randomness in QM). For each "observable" you want to measure
(position, momentum, spin), there is a specific linear operator associated
with it. For instance, in position-space, the position operator is just the
identity operator.

If you haven't taken linear algebra, I can clarify this further.

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FrankenPC
I have a suspicion what you are trying to tell me is at the root of this. But,
no I do not remember any complex math. Yes, I took multiple calculus classes
in college, but I don't remember any of it. That was decades ago.

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abritishguy
The title is too bold, after seeing a claim like that the video is
disappointing (doesn't really explain much just states things).

The video is very good and interesting but certainly not "The most awesome
animation i will ever see". Upvote video, downvote title.

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shire
This stuff just blows my mind. I really want to get into Quantum Computers but
I don't think I'll be able to wrap my mind around it. It's really amazing how
much stuff we don't know in this universe.

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elboru
I'll oversimplify my question, how will be possible for us to get useful
information from a system based in randomness?

For my understanding, I can ask a quantum system: is this a cow? and it'll
answer:

1\. Yes

2\. No

3\. 72

~~~
geuis
The video really doesn't do a good job of explaining how a quantum computer
can work. They just hit the basic high points because this video is intended
for a general rather than specific audience.

A qubit exists in multiple states until measured. When you entangle multiple
qubits together, each one increases the power of the system exponentially.
Quantum computers are only good for certain kinds of problems like factoring
large prime numbers. Luckily quantum systems are good at things classic
computers aren't, and vice versa. You have to state your "question" in such a
way that it causes the overall state of the quantum system to resolve on your
answer.

I know my answer isn't much better. I only have a high level understanding
with none of the math behind it. There's good videos if you search and watch a
few times to get the concepts.

I can't remember the exact scenario that was explained to me, but it was
something like this. Imagine the Traveling Salesman problem. You want the best
route. But with a regular computer, it might take thousands of years to
calculate the best route. With a quantum computer, if you structure the
measurement correctly (this is how you "program" a quantum computer) then only
the best route is what the system decoheres to.

There was also something about light and how if you had the right filter, only
the answer gets through. I really forgot the details of that one, but maybe
it'll help.

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ssivark
To substantiate your point: Essentially, quantum computing is useful for
problems that could take help from a large degree of parallelization. _Quantum
mechanics has an inherent parallelization at it heart_. Otoh, simulating that
amount of parallelization with classical resources will be exponentially
complex.

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takeoutweight
This is a VERY common misconception, and is not a good intuition for how
quantum computing is different than classical.

A better intuition, in my opinion, is computing with "un-flipped" coins,
except with funny complex-valued probabilities of heads vs tails instead of
real values between 0.0 and 1.0. Because the probabilities are complex-valued,
they interact in ways that can seem a bit counter-intuitive, (it makes sense
it is counter-intuitive, because how many quantities do we deal with day-to-
day that are described with complex numbers?) These interactions can provide
_some_ algorithmic speed up on _some_ nicely structured problems.

It's not always obvious what these problems are, and it doesn't relate to do
whether the problem is highly-parallel. My intuition for "quantum-friendly" is
if a problem involves some kind of periodicity, or Fourier-like frequency
analysis, then perhaps you'll see a quantum speedup. But it's important to
remember the coins don't somehow "try every flip possible" to find the answer
you're looking for -- if they did that you'd be able to solve NP problems in
constant time.

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ssivark
I agree that the superposition with complex probability amplitudes
(fundamental quantum randomness) is different from just _statistical
randomness_ of trying every possibility. I fact, this is one of the notions
which is being used to verify whether the D-Wave computer (as a black box) is
"truly quantum".

1\. I don't quite see what periodicity has to do with the speedup. Could you
elaborate on that?

2\. _"...if they did that you'd be able to solve NP problems in constant
time."_ Again, please explain. In QM, the answer you calculate is actually the
weighted sum over all possibilities... so the way I see it, it does cover
every possible path. That answer might not be useful in telling you which is
the shortest single path, but if you were an electron traversing a graph and
wanted to optimize your travel, you'd do exactly what the quantum computer
says and traverse all paths! The classical notion of "best path" might be
trickier to deduce from a quantum computer. Maybe I'm addressing something
tangential to what you're saying.

~~~
takeoutweight
Re 1. Periodicity: The quantum Fourier transform is an operation that reflects
information about a quantum state into the the phases of the amplitudes.
Quantum phase information is exactly where quantum computing differs from
classical probabilistic computing, so it makes sense that this technique might
show up in places where quantum computing beats classical. For example: Shor's
factorization of integers makes direct use of the quantum Fourier transform. I
mention periodicity only as an example of a sort of problem where the Fourier
transform might be useful. This is as an alternative intuition to what quantum
computers are "good at" to combat the notion that "quantum computing is good
for parallel problems."

Re 2. "covering all paths": I don't have a problem interpreting quantum
superposition as inhabiting all possible states. However, classical
probabilistic computing also can be interpreted this way: an un-flipped coin
is both heads and tails until the flipping happens. But probabilistic
computing doesn't give us faster-than-classical speedup, therefore it's not
just the "existing in all states at once" that buys us the speedup: it's the
unique kind of math we can do on these states because our amplitudes are
unitary-complex, not positive-real.

You understand this distinction, so I perhaps shouldn't have adopted the tone
I did (sorry!). The leap between "superposition involves a complex-valued
combination of several states" and "tries all answers at once so is very fast"
is a very common leap made in popular science articles and is the kind of
misconceptions that I think the "quantum computers are good at parallel"
intuition encourages. Just because a quantum state is a superposition doesn't
mean we get to, for free, observe and evaluate all those states and pick out
the one we like. We can _sometimes_ arrange things in such a way that the
quantum phases interact non-classically to leverage the structure of _some_
problems to reveal information that isn't available to classical algorithms.

Periodicity information, via the quantum Fourier transform, is an example of
one way to arrange things to extract information that would be more expensive
to calculate classically.

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bhauer
As soon as I saw the page, I thought, "Is this a Jorge Cham [1] project?" Sure
enough, it is! Awesome.

[1] [http://www.phdcomics.com/](http://www.phdcomics.com/)

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fc2
Probably a dumb question, but how can we know for sure that it's intrinsic
randomness? Maybe we just haven't figured how it really works?

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IanCal
It's not a dumb question, that's what we should start by assuming because it
makes a lot more sense. This is referred to as "local hidden variables".

Bell's theorem
([http://en.wikipedia.org/wiki/Bell's_theorem](http://en.wikipedia.org/wiki/Bell's_theorem))
and the tests around it show this to be false, things just don't act like
there's a variable we simply don't know.

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HCIdivision17
That is exactly the experiment I couldn't remember the name of and tried to
ramble my way around it via another route.

This is the clearest way to understand that it must be truly random and not
just hidden ignorance of state, and has been tested to a remarkable degree of
accuracy.

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ORioN63
The black box thing, really looks like a monad.

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takeoutweight
You may be interested in the category-theoretic presentation of quantum
theory. Turns out we're in more of a monoid than a monad! :)

[http://math.ucr.edu/home/baez/quantum/](http://math.ucr.edu/home/baez/quantum/)

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ssivark
Here's some recent work, trying to understand QM using notions from category
theory -- [http://arxiv.org/abs/1303.6917](http://arxiv.org/abs/1303.6917)

