
Performance of quantum computer no better than ordinary PC, say analysts - aasarava
http://www.csmonitor.com/Science/2014/0619/Performance-of-quantum-computer-no-better-than-ordinary-PC-say-analysts
======
Xcelerate
The title of the article "Performance of quantum computer no better than
ordinary PC" is just flat out wrong. The real issue that physicists are
concerned about is whether the D-wave machine is a quantum computer or not.
The issue is not whether quantum computers are "better" than classical ones.

Assuming the correctness of quantum mechanics, a quantum computer WILL be
faster than classical computers for specific classes of problems,
exponentially faster in some cases. This has been mathematically proven.
(EDIT: No it hasn't apparently. See Michael Nielsen's response below.)

~~~
michael_nielsen
Your first paragraph is quite right. But the second paragraph is wrong: for
(non-oracle) problems quantum computers are merely suspected to be faster in
principle than conventional classical computers. It's never been proven.

~~~
Xcelerate
I guess I'm confused then. I thought that quantum algorithms had been proven
to be faster than the best known classical algorithms (for instance, Shor's
factorization), assuming the correctness and completeness of the postulates of
QM. In other words, the order of the algorithm is derivable from the
postulates. Is that not the case?

I think there's a few people (t'Hooft if I remember correctly?) that believe a
quantum computer will never be able to _experimentally_ demonstrate this
speed-up because some aspect of quantum mechanics is incomplete/incorrect.

Edit: Here's the thread I was thinking of where t'Hooft and Shor discuss this
[http://physics.stackexchange.com/questions/34217/why-do-
peop...](http://physics.stackexchange.com/questions/34217/why-do-people-
categorically-dismiss-some-simple-quantum-models)

~~~
michael_nielsen
Shor's algorithm is exponentially faster than the best existing classical
algorithm for factoring. But that doesn't mean there's not an exponentially
faster classical algorithm which would be comparable in speed to Shor's
algorithm. Some noted experts believe this is possible, and even likely.
Richard Lipton believes it, and claims that Peter Sarnak does too:

[http://rjlipton.wordpress.com/2013/04/27/sex-lies-and-
quantu...](http://rjlipton.wordpress.com/2013/04/27/sex-lies-and-quantum-
computers/)

Yes, 't Hooft and others, including Leonid Levin and Oded Goldreich, believe
that quantum computers won't function because of quantum mechanics breaking
down. See, e.g.:

[http://www.wisdom.weizmann.ac.il/~oded/on-
qc.html](http://www.wisdom.weizmann.ac.il/~oded/on-qc.html)

[http://www.cs.bu.edu/fac/lnd/expo/qc.htm](http://www.cs.bu.edu/fac/lnd/expo/qc.htm)

~~~
axilmar
Aren't algorithms like Shor's essentially faster than others in their
respected categories due to parallelization?

I.e. isn't quantum superposition just a form of parallelization? if the
quantum step in Shor's algorithm is replaced with a classical step executed in
parallel in many cpus, then the speed up would be of the same order of
magnitude as when using a quantum step.

~~~
keeperofdakeys
Not really, if that was the case, quantum computers really would be faster
than classical computers. A qubit is basically a normal bit, with probability
a that it's in the 0 state, and b that it's in the 1 state, with a^2 + b^2 =
1. If you do some clever maths, you can get these probabilities to cancel out,
and give you something that will collapse to the proper state when read.

[http://en.wikipedia.org/wiki/Shor's_algorithm#Finding_the_pe...](http://en.wikipedia.org/wiki/Shor's_algorithm#Finding_the_period)

Shor's algorithm is still very theoretical though, we need to actually build a
quantum computer to run it first.

------
dm2
Here is another article today bashing the D-Wave Two computers:
[http://www.wired.com/2013/06/d-wave-quantum-computer-
usc/](http://www.wired.com/2013/06/d-wave-quantum-computer-usc/)

These machines are testing a completely new computing concept. We're trying to
learn how they work so that we can apply that to building machines that can
actually outperform traditional computers, and once that's achieved then the
potential benefit will be enormous.

According to [http://www.dwavesys.com/d-wave-two-
system](http://www.dwavesys.com/d-wave-two-system) as you scale up qubits the
power demands do not increase. That alone has huge potential.

Maybe new materials need to be developed to see the full potential of these
computers, lots of research needs to be done. Why are all of these articles so
pessimistic?

NASA, Lockheed, and Google purchasing these multi-million dollar machines
should indicate that there is at-least some potential and value in the D-Wave
computers, even if in the end they just learn what D-Wave did wrong.

[https://www.youtube.com/watch?v=CMdHDHEuOUE](https://www.youtube.com/watch?v=CMdHDHEuOUE)

"We don't know the best questions to ask the quantum computers, that's what
we're trying to find out now."

~~~
JoeAltmaier
So negative because there's still a non-zero probability that its not going to
work at all, ever. That there's something about quantum computing we've
figured out wrong, and no benefit is possible.

~~~
VonGuard
They're also negative because no one has actually proved that the D-Wave is,
in fact, doing any actual quantum computing. That's been the issue all along
with D-Wave: no one can prove that it's really doing anything with those
qbits.

~~~
dm2
It depends on your definition of quantum computing, the D-Wave does
[http://en.wikipedia.org/wiki/Quantum_annealing](http://en.wikipedia.org/wiki/Quantum_annealing)
and has been faster in some applications.

[https://plus.google.com/+QuantumAILab/posts](https://plus.google.com/+QuantumAILab/posts)

They're just getting started, this field is very new, very difficult, and
there are a lot of very smart people trying to answer the
what/how/why/where/when of "quantum" computers.

The hardware outperforms off-the-shelf solvers by a large margin

"In an early test we dialed up random instances and pitted the machine against
popular of-the-shelf solvers -- Tabu Search, Akmaxsat and CPLEX. At 509
qubits, the machine is about 35,500 times (!) faster than the best of these
solvers. (You may have heard about a 3,600-fold speedup earlier, but that was
on an older chip with only 439 qubits." [1]

[1]
[https://plus.google.com/+QuantumAILab/posts/DymNo8DzAYi](https://plus.google.com/+QuantumAILab/posts/DymNo8DzAYi)

~~~
foobarqux
> The hardware outperforms off-the-shelf solvers by a large margin

But it doesn't outperform classical algorithms specifically written to solve
the types of problems that the D-Wave machine accepts.

~~~
VonGuard
Yeah, and I've never seen anyone outside D-Wave show these types of speedups.
Last article I read, in fact, was struggling really hard to even show that
D-Wave was the same speed as traditional algos.

------
icegreentea
This is the paper in question:
[http://www.sciencemag.org/content/early/2014/06/18/science.1...](http://www.sciencemag.org/content/early/2014/06/18/science.1252319)

Which appears (I can't tell, paywalled) to be an update or this one:
[http://arxiv.org/abs/1401.2910](http://arxiv.org/abs/1401.2910)

I don't have to time to read the article properly right now, but doesn't look
like they did anything ridiculous in their choice of testing algorithm.

------
robgibbons
Firstly, this is the first of a new class of computers, so I am not surprised
it may be slow at first. Secondly, this machine uses a relatively small number
of bits, compared to a classical computer. Lastly, they're throwing classical
problems at a quantum processor and wondering why it's not magically faster. I
would like to see how an optimized quantum algorithm performs.

------
bwy
A few other comments have pointed it out already, but how does this comparison
actually mean anything? For example, multiplication with the first hole-punch
computers was no faster than multiplying by hand and paper (probably slower),
but look where we are now!

------
mpthrapp
The title is slightly misleading. This article is only talking about the
performance of one specific quantum computer. (The D-Wave Two).

~~~
VonGuard
Are there others that you can buy? As far as I know, D-Wave is the only one
out of the lab. I'm totally a D-Wave skeptic, but I do have to state that the
scientists who tested this thing sound like idiots, unless they really did try
quantum annealing. If the D-Wave guys say they tried the wrong set of
equations for the device, I'm inclined to believe them.

Even though I don't even believe this thing is really a quantum computer,
there are much more specific and defined ways to poke holes in D-Wave than
just by running some tests and comparing speed. D-Wave has, very specifically,
targeted this machine at one function:
[http://en.wikipedia.org/wiki/Quantum_annealing](http://en.wikipedia.org/wiki/Quantum_annealing)

Unless you're doing quantum annealing on the D-Wave, it might as well be a
toaster, an Android phone, or a Windows PC. It's made to do only quantum
annealing.

Did D-Wave 2 somehow expand its capabilities? I'm basing this on D-Wave 1
knowledge, but I cannot imagine anything really changed except the number of
Qbits inside the thing.

All this having been said, the D-Wave has yet to prove that it is even doing
Quantum Annealing, let alone that it's faster than a traditional computer at
this problem set. No one has shown it to be faster than traditional outside of
D-Wave.

~~~
shoyer
Yes, of course they did what D-wave calls quantum annealing. As you say,
that's all the D-Wave machines can do.

The D-Wave executive's claim that the problems used in the study are "not at
all the right choice for probing a quantum speedup" is in reference to the
specific subtypes of quantum annealing problems that this study looked at, not
whether they were doing quantum annealing at all.

------
yellowapple
What's the definition of "ordinary PC" being used? Not finding an answer to
that in the article, which makes it difficult to get an idea of what the
researchers are comparing this thing to.

~~~
Bakkot
One which doesn't exploit quantum-mechanical principles of computation.

~~~
yellowapple
So is that a brand-new Dell XPS machine? Is that my aging-not-too-well Compaq
Presario 1210 with a Pentium? Is that a Macbook Air? Is that one of Oracle's
$200,000 M-series servers? Is that some beige-box piece of junk that the
researchers found in their neighbor's backyard? The category "personal
computers which do not exploit quantum-mechanical principles of computation"
is so mind-bogglingly vast that such a description alone is hardly useful.

------
ColinWright
Also:

[https://news.ycombinator.com/item?id=7917482](https://news.ycombinator.com/item?id=7917482)

[https://news.ycombinator.com/item?id=7917749](https://news.ycombinator.com/item?id=7917749)

------
owenversteeg
Original article: [http://www.livescience.com/46414-first-quantum-computer-
no-f...](http://www.livescience.com/46414-first-quantum-computer-no-
faster.html)

------
MrBuddyCasino
Previously:
[https://news.ycombinator.com/item?id=7917749](https://news.ycombinator.com/item?id=7917749)

------
michaelochurch
Weird title. If someone built a quantum computer that performed remotely as
well as a 2014-era classical machine, that would be huge news. Quantum
computers are _really_ hard to build.

