
Quantum computing for everyone - ntoshev
http://michaelnielsen.org/blog/quantum-computing-for-everyone/
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nico
I think the author didn't achieve his own goal: _“Can you give me a simple,
concrete explanation of how quantum computers work?”_

The explanation, even though it doesn't involve any complicated math or
technical stuff, is long, convoluted and full of excuses of why there can't be
a simple explanation.

~~~
ntoshev
The excuses are part of the explanation ;) I don't know why people expect to
have metaphors from the macro world that can be readily applied to quantum
mechanics - the whole particle-wave duality for example seems just an attempt
to explain experimental results / equations with two partially broken
complementary metaphors - just because this is all we have at hand in the
everyday world...

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roundsquare
Question for someone who knows more about this:

Is this the right way to think about it? What we need to do to write a
"program" to solve a problem is:

1) Find a set of gates to setup the atoms so that they have the right input.

2) Find a set of gates that uses this input to solve the problem.

I know very little about quantum computation so any help is appreciated...

~~~
pmjordan
Take a look at Shor's algorithm[1] for factoring integers on a quantum
computer. It's definitely not for the faint of heart, but it should show you
what goes into creating an algorithm for a quantum computer. People complain
about concurrent programming, they have another thing coming if quantum
computers become commonplace. I suspect it will be more like programming an
FPGA than a CPU.

In any case, as I understand it, you will need to add a third step, which is
to extract the correct solution from the system's state, which is also non-
trivial.

[1] <http://en.wikipedia.org/wiki/Shors_algorithm>

~~~
roundsquare
_In any case, as I understand it, you will need to add a third step, which is
to extract the correct solution from the system's state, which is also non-
trivial._

Good point, I didn't think about retrieving the answer.

Is that something can be done with gates or is there another method?

Edit:

 _People complain about concurrent programming, they have another thing coming
if quantum computers become commonplace. I suspect it will be more like
programming an FPGA than a CPU._

Seems like you might be right in the short term, but I wonder if it will be
true in the long term. E.g. once you get methods for doing a bunch of standard
stuff you can put them together. Not sure if that works for quantum computing
but maybe some other way of abstracting out the details.

~~~
pmjordan
_Good point, I didn't think about retrieving the answer. Is that something can
be done with gates or is there another method?_

My (simplistic) understanding is this: you need to think of the "registers" in
a qc as not storing one value but essentially a probability distribution, so
your operations are really convolutions of those distributions. "reading" from
the register is a measurement, which collapses the wavefunction into a set of
states which happen to share the value for the attribute you are measuring.
The outcome of the measurement is random but governed by the probability
distribution that the system has for that attribute at that point. You need to
make the desired answer stand out in that distribution (i.e. more likely to
come up), otherwise you won't have gained anything vs a classical computer.

 _Seems like you might be right in the short term, but I wonder if it will be
true in the long term._

I'm having a hard time understanding this stuff at the micro level, whereas
assembly language of a classical computer is trivial in comparison. I made the
FPGA and concurrency analogy because you have to account for global state, not
just the state of registers touched by one instruction. Except the "global
state" in a qc is a different beast entirely. I guess I don't have a clue what
these abstractions might look like so I'm going to accept that you may well be
right.

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amichail
Related: I'm still not satisfied with the answer as to why quantum
cryptography's eavesdropper detection -- not possible with a Turing machine --
can't be embedded in a model of computation that goes beyond Turing-
completeness:

<http://news.ycombinator.com/item?id=955079>

~~~
mkyc
Because it is possible with a Turing machine.

<http://en.wikipedia.org/wiki/Quantum_Turing_machine>

[http://74.125.155.132/search?q=cache:CgFC04CcivAJ:folk.uio.n...](http://74.125.155.132/search?q=cache:CgFC04CcivAJ:folk.uio.no/ovrum/articles/deutsch85.pdf)

The idea (iirc, and poorly translated into more familiar vocabulary) is that
when we're running a quantum Turing machine on a regular machine, the data on
the tape can be "quantum encoded". There is a specific series of steps for
"decoding" it, but it's a destructive operation: decoding the data screws up
the data. Decoding it a second time gives you broken data, and you know that
it's broken.

(You might wonder, why don't we just write a Turing machine that can decode
the data without destroying it? Well, you're free to do so, but then it's no
longer a quantum Turing machine running on a Turing machine - it's no longer
the same as the physics, can't be implemented directly in quantum physics, and
therefore would be both slow and potentially have a tape with more slots than
atoms in our universe.)

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danbmil99
If, as the poster claims, pretty much everything we care about in the real
world can be simulated with classical computers, what exactly will a quantum
computer do that is in any way useful to anyone other than a scientist working
on quantum computing or some other research related to quantum mechanics
itself?

~~~
roundsquare
Solve "big" problems much much much faster.

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inc
What are the obstacles for a hobbyist who wants to build a simple quantum
computer?

~~~
chrjozefharibo
For solid state qubits (spin/flux) you need:

* e-beam lithography: expensive machine

* high quality materials (GaAs 2DEG, or reproducible Josephson junctions): you need collaboration with other scientists

* a huge supply of He4: money

* a dilution or He3 fridge with He3 in it: more money

* a lot of low-noise amplifiers and electronics: collaboration and a lot more money

And a lot of time and persistence.

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tybris
Sooo... when can I launch a quantum EC2 instance?

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tybris
What was I thinking joking on an American website? Sorry guys.

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onoj
The guy seems like a quantum fanboy who does not understand what he is talking
about. Isn't data management separate theory from the hardware - ie: using
smaller objects to make computers does not change the management of the data.

~~~
pmjordan
Quantum computing has nothing to do with making computers smaller.

~~~
onoj
Yes, but coming up with a non binary data management system (ie: one that uses
a bit in four states) requires a whole new language of machine code before you
can get the quark to talk. Making one bit of info is still thirty years away
from a functioning device

