

Testing a New Class of Speedy Computer - tvladeck
http://www.nytimes.com/2013/03/22/technology/testing-a-new-class-of-speedy-computer.html

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troymc
D-Wave does a nice dog-and-pony show, but I won't be cheering until they
actually do one of the long-promised "holy grail" things that quantum
computers are supposed to be able to do.

Like what? Like running Shor's Algorithm (for integer factorization) on a big
integer, in the expected amount of time.

<https://en.wikipedia.org/wiki/Shors_algorithm>

(and I will be cheering, because they're based in Burnaby, where I live.)

~~~
Ygg2
I don't think you can run that Algorithm on adiabatic quantum computer. From
what I recall quantum adiabatic chips use hills and valleys optimization, but
they aren't generalized quantum comptuer.

[http://www.newscientist.com/article/dn23251-controversial-
qu...](http://www.newscientist.com/article/dn23251-controversial-quantum-
computer-aces-entanglement-tests.html?full=true)

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r2
Some USC scientists recently analyzed the performance of a D-wave system and
found that "quantum annealing is indeed being performed by D-Wave One":
[http://www.pppl.gov/events/adiabatic-quantum-computing-d-
wav...](http://www.pppl.gov/events/adiabatic-quantum-computing-d-wave-one)

So their computer does behave quantum mechanically. But I can't find any
evidence that they have shown entanglement, which is necessary to realize the
full promise of quantum computing, see e.g.
<http://www.scottaaronson.com/blog/?p=954#comment-40364>

~~~
tiziano88
I think they only do _adiabatic_ quantum computing, which is not what most of
the people mean when speaking about quantum computing (it does not involve
entanglement, only quantum annealing). Edit: Wikipedia article:
<http://en.wikipedia.org/wiki/Quantum_annealing>

~~~
cbennett
this is correct. for those with a hazy understanding of quantum mechanics- or
to lazy to read link- the adiabatic process basically adds an extraneous
(kinetic) part to the Hamiltonian and then the commutation of these properties
allows for a superior process than thermal annealing. D-Wave folks would like
us to believe that this imitates the vast majority of computation improvements
expected in QC, and it may yet create some performance improvements.

However, in 'true' QC, there is something very physically different happening;
we expect to see universal superposition of Hamiltonian states (entangled
vector bases), which allows for some remarkable parallelism in a gate
architecture that mirrors programmable (classic) computers. Specifically, we
would be able to perform an operation on 2^N different numbers with just one
calculation, while in classical computing such a computation would actually
require 2^N separate (sequential) iterations. DWave systems, although it has
produced some evidence of engtanglement, could not achieve anything on this
magnitude of computational efficiency/parallelism. Footnote: as promising as
the big picture of recombining said superpositions and 'hacking the
multiverse' (a la Deutsche) is, fragility & decoherence of said states is an
extraordinary barrier to overcome in order to achieve a 'true' QC in the near
term or even medium term (as some critics as Wolfram would argue)...

~~~
gizmo686
Your explanation of 'true' QC seems misleading as well. We can (and have)
constructed the quantum gates necessary to build a computer with the classical
gate structure. It is true that in doing so, if we make the input a
superposition of states, then the output will be a superposition of the states
that would result from passing in each individual state into a classical
computer. However, this is not sufficient for us to actually take advantage of
the massive parralelization. Lets assume you have a N quibit input, which is a
perfect superposition of all 2^N possible states, and as many internal quibits
as you want. You have some boolean function, f(n), that is true for exactly 1
number (and that number fits in N bits). If you tried applying programming
your QC to apply f(n) to the superposition, then after you do that, you will
have a superposistion of 2^N-1 0s, and one 1, all with an equal probability.
When you go to measure it, you will read either 0, or 1. Not very usefull.

You can fix this problem using N+1 quibits, and returning the input number
along with the answer. Now, you have 1 state where the 'answer' quibit is
true. When you go to measure the system, you have a 1/(2^N-1) probability of
seeing that state. If you see another state, then you have collapsed the
superposition and need to redo the computation. This is equivalent to just
randomly picking an input in the first place. Quantom mechanics does not allow
a way of taking a system and making one state more probable based on its value
(IE. guaranteeing that the one you measure is the one marked 'true'). However,
their are some quantum mechanical mechanisms that can provide provable
improvement.

One such algorithm is Shors, which allows efficient factorization of prime
numbers.

In my opinion, their is a far more interesting result I have seen, which is a
general algorithm that can take any problem with a solution space of 2^N, and
solve it in 2^(N/2).

Sadly, it has been a while since I dabbled in quantom computing so I do not
recall the details of that, and I very well might have gotten stuff wrong to.

I am however becoming convinced that P/=NP is a physical law of the universe.

EDIT: I just read farther down the thread. I think the algorithm I was
referring to is Grovers search algorithm which was mentioned by zaptheimpaler.

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shard
Paywall-less Google link to article:
[http://www.google.com/url?sa=t&rct=j&q=&esrc=s&#...</a>

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fiatmoney
The folks at D-Wave have published some interesting & perfectly legitimate
papers on techniques for optimizing parallel tempering simulations (including
on non-quantum computers, but especially helpful if you have a quantum
computer handy for quantum PT, which is how they vaguely describe their actual
setup). That suggests to me that at the very least they have some legit people
who are good at solving stochastic optimization problems & are not fraudulent.

~~~
Ygg2
Newscientist did report they have proved that their computer does work with
entagled particles, so its an quantum computer and not a sham.

~~~
greenmountin
This is false. I guarantee you, scientists would be very interested if they
actually demonstrated entanglement. For example, in the slides of the linked
talk, the presenter goes over a very general measure of entanglement which
allows you to construct after-the-fact a "witness" of entanglement. It is
respected theory, and a credible last-resort approach; no numbers. His latest
preprint[1] doesn't have a clear claim either. The Newscientist section refers
to a conference 2 weeks ago, without published abstracts, procedings, or
video. I'll believe it when I see it (there's no indication Aram's quote is in
context, but maybe he has).

The first thing any qc lab does when they get their second qubit is make an
entangled state and measure Bell Inequality violation. D-wave's lack of
interest in this or in running Shor[2] is very telling. Everybody wants to run
Shor, it's where the money is.

[1] <http://arxiv.org/abs/1212.1739>

[2] [http://dwave.wordpress.com/2013/03/20/computational-
universa...](http://dwave.wordpress.com/2013/03/20/computational-
universality/#comments)

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niggler
I remember many people, especially Scott Aaronson, questioned D-Wave back when
I was in school. Has anything changed recently regarding the community's
opinion?

~~~
AndrewKemendo
Scott's visit to D-Wave shifted his opinion. He obviously can't come out and
say he changed his mind as a professional skeptic but it comes about as close
to saying as much.

<http://www.scottaaronson.com/blog/?p=954>

~~~
gjm11
This seems to me a very inaccurate and unfair characterization, especially the
insinuation that he's dishonestly suppressing his newfound belief in D-Wave
because he's a "professional skeptic".

Aaronson is clearly still unconvinced that D-Wave's machine actually does
anything "sufficiently quantum" to get any speedup over what you can do
classically. He says:

 _D-Wave does have something today that’s more computationally-useful than a
roast-beef sandwich; the question is “merely” whether it’s ever more useful
than your laptop._

and:

 _It remains true, as I’ve reiterated here for years, that we have no direct
evidence that quantum coherence is playing a role in the observed speedup, or
indeed that entanglement between qubits is ever present in the system._

On the other hand, he is willing to say that D-Wave have indeed made a machine
that does Something Interesting, namely giving approximate solutions to a
particular mathematical problem.

So it looks to me as if he (1) has changed his mind about the things he's been
given actual evidence for, (2) is perfectly willing to say so despite his role
"as a professional skeptic", and (3) hasn't so far been convinced that D-Wave
have an actual useful quantum computer doing quantum-computer things that you
couldn't do just as well with a classical machine. Which is approximately the
exact opposite of what you say.

~~~
AndrewKemendo
>especially the insinuation that he's dishonestly suppressing his newfound
belief in D-Wave because he's a "professional skeptic".

That wasn't my intent; moreso I intended to portray that he is emphasizing,
and greatly warranted in this case, scientific skepticism paraphrasing his own
terms. I don't think that is the opposite.

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MichaelApproved
How does something like this affect bitcoin?

Putting the cost of the machine aside, would a computer like this be able to
mine coins at high speeds using relatively little electricity. Seems like it
could crush the market with inflation.

~~~
Jach
It's an interesting technical question: analogous to the quantum fourier
transform, is there an algorithm only quantum machines can run that computes
sha-256 hashes faster? I don't have the answer. (I know sha-256 is O(N)
normally.)

As for bitcoin, though, I don't think there's too much to worry about. If a
quantum algorithm grants a complexity speed up (or some sort of deal-breaking
cryptographic weakness is discovered in sha-256 for that matter), I imagine
the network could agree to update to a new protocol using a different hash, if
that was the proper solution. If a quantum computer just gave integer-factor
speedups such as ASICs are currently doing to GPUs and FPGAs, then I suspect
by the time a quantum computer is made that can efficiently mine bitcoins,
enough can be made at once that, just as we're seeing with ASICs, quite a lot
of miners will grab them (avoiding a 51% issue) and the difficulty will simply
rise.

Edit: <https://bitcointalk.org/index.php?topic=78693.0> tells me
<http://en.wikipedia.org/wiki/Grover%27s_algorithm> would work, making the
algorithm O(sqrt(N)) and effectively making the problem of finding a sha-256
hash into finding a sha-128 hash.

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nnq
> It could be possible, for example, to tell instantly how the millions of
> lines of software running a network of satellites would react to a solar
> burst or a pulse from a nuclear explosion

Can anyone explain in understandable terms how a quantum computer will help
solve this type of problem? Or is the article's author just dumping random
affirmation to hype things up?

~~~
zaptheimpaler
Theres some truth to it. Despite how revolutionary quantum computing _could_
be, there still aren't a lot of quantum algorithms that are asymptotically
faster than their classical counterparts. The two important ones are Shor's
factorization, which can factor an integer in O((log n)^3) time, and Grover's
Search, which can search an unsorted collection in O(sqrt(n)) time.

Grover's search is pretty amazing though - a lot of problems can be reduced to
some form of search in the solution space, and sqrt(n) search will let us say
"fuck it, lets just brute-force it" to much bigger search spaces. I could see
that being applied to the example you mentioned - searching through a list of
possibilities very quickly.

