How much faster is a quantum computer than your laptop?

"OK, I promise not to get out over my skis. I wouldn't know how."

Me too. I've been a skier for over three decades now (not all the time) and can't quite work out what that means. From the context I think: "Let's not get ahead of ourselves" - a far more logical and understandable phrase!

Re: Quantum computers

@"how do you go about checking the machine got the right answer"

DWAVE is an analogue computer running a sort of annealing algorithm configured using circuits and magnetic coils.

Lots of problems, that can be described as annealing problems, already have faster digital algorithms to solve them. So you can run the algorithm in a DWAVE as an annealing problem and in a computer using a different method, and see if the computer result is more optimal (i.e. better) than the DWAVE. So you know the DWAVE didn't return the optimal result.

IBM did this to diss DWAVE, pointing out that actually digital computers running digital algorithms are faster than DWAVE running annealing. Whereas DWAVE promoters generally compare DWAVE annealing to simulated annealing on a digital computer to claim the speedup. But that would only be faster for problems where no suitable digital algorithm exists.

The other way its done is to run an annealing algorithm in DWAVE and simulated annealing in a computer. And simply run the computer for longer till its result is better than the DWAVE. The computer doesn't need to get a perfect result, because we don't know the perfect solution. It only needs to get a *better* solution than the DWAVE to know the DWAVE didn't find the perfect solution.

Feynman, the physicist who coined the phrase Quantum Computer, did his work in the 50s and 60s, where computers were often analogue computers. And he imagined such a computer that could work instantaneously (using entanglement) and always get the optimal solution (using superposition). A sort of brute force that tries every possible solution.

DWAVE isn't that. It's a sort of annealing circuit.

It's also clearly not a quantum computer in the Feynman sense. No superposition and no entanglement. It's very clever marketing though.

Re: Quantum computers

>how do you go about checking the machine got the right answer if it takes a several hours to run the program? It would take years on a regular supercomputer.

Checking a single answer can be done near instantly on a classical computer, for many types of problem. The issue is, you have 10^daft possible solutions to choose from - and it is this area that true quantum computers will excel (if someone ever manages to build one).

If you are still doubtful about your quantum algorithms, you could test them with problems to which you already know that answer (because you have constructed the problem for this purpose, from the answer and working backwards)

Re: Wrong question

Of course, quantum computers are fantastic for gaming. No more agonizing about which path to take or which weapon to use, you can make all possible choices at the same time and then select the best outcome.

Google a bit about D-Wave, and you'll understand why a lot of people are sceptical about their claims. So far they've shown no strong evidence of achieving quantum properties in their computers, and their test results have been outperformed by optimized software running on classical computers.

"My, what a lot of cynical people we have with us today."

And it's a bloody good thing too.

Cynic (noun):

- "a person who believes that people are motivated purely by self-interest rather than acting for honorable or unselfish reasons." So basically anyone with actual functioning eyeballs.

- "a person who questions whether something will happen or whether it is worthwhile." I certainly hope so - when exactly did questioning things become unfashionable outside countries with "beloved leaders"...?

Re: But...

Yes

and No

Re: Time to solve one of those "10^77 years" IT Security Cracking problems

>Past time for DARPA to set the rules for a formal competition.

No, no it isn't.

Breaking conventional cryptography requires a class of quantum computer that Shor's algorithm - which D-Wave has never claimed to be capable of.

Quantum computers than can run Shor's algorithm have been built in the lab, but only up to a few qubits - far too small to be of any use in factorising large numbers (and thus breaking encryption as we now use it). In the event of researchers making significant progress in this area, DARPA or the NSA would not have to hold a competition - they'll already have been paying for the lab. In any case, it would be trivial for even an amateur mathematician to quickly determined if it worked or not.

The D-Wave is a different beast. The smoke and mirrors to which you refer are due to the difficulties in finding fair tests. Still, there's enough uncertainty for Google to take a punt on it, as they should if there is even a small chance it can be made to work for them, finding low points in fitness landscapes.

Re: I don't claim to understand it, but ...

My recommendation is to read articles like this carefully, with the understanding that pretty much every claim in them is wrong.