Programmable Quantum Computer Created

An anonymous reader writes "A team at NIST (the National Institute of Standards and Technology) used berylium ions, lasers and electrodes to develop a quantum system that performed 160 randomly chosen routines. Other quantum systems to date have only been able to perform single, prescribed tasks. Other researchers say the system could be scaled up. 'The researchers ran each program 900 times. On average, the quantum computer operated accurately 79 percent of the time, the team reported in their paper.'"

Re:Ha ha

[Anonymous Coward]

not if they are parallel. computations that are done on each node where the majority of the answer is the right answer (as long as it's accurate past 50%). won't increase speed but will increase accuracy.

Re:79% accuracy ...

[chhamilton]

No, it's actually a perfectly reasonable idea. Consider running the device (n+m) times. The probability of it being right n times and wrong m times is given by:

P(n,m) = (n+m)!/n!/m! 0.79^n 0.21^m

Now consider the probability of it being right (majority has the right answer) out of 2n+1 trials. This is the given by:

S(n) = sum( P(n+1+i,n-i), i=0..n )

This can be simplied to a closed form using Legendre and gamma functions, but that's kind of messy and it's far easier to just plug in values and do the summation. As it turns out, doing the experiment 15 times and taking the majority (plugging 7 into S(n)) will give you the correct answer 99.4% of the time. Doing things 35 times gets you to five nines of accuracy... completely reasonable in my books.

Re:79% accuracy ...

[Frequency Domain]

79% accurate. That's pretty useless.

Not useless at all, just have it solve the same problem 5 or 15 times and go with the answer that it gives most often. Plus, for some problems it's much easier to verify an answer than to come up with it -- for those problems, just pair it with a normal computer to check the answers, and keep trying until it says the answer is right.

One of the classic examples of that last one is prime factorization. In general it's very hard to come up with the two primes that were multiplied to create a very large number, but if the quantum computer coughs up a candidate it's downright trivial to check whether that's a solution.

Re:79% accuracy ...

[xZgf6xHx2uhoAj9D]

Actually quantum computing is, by design, probabilistic. Every specifically quantum algorithm (even Shor's infamous factoring algorithm) gives incorrect results by design for the simple reason that it's really not possible to have quantum algorithms which succeed all the time (unless you forgeo their quantum properties). So long as the probability of a correct answer is strictly greater than 0.5, however, one only has to repeat the computation a constant number of times to get the probability of success arbitrarily close to 1.

Re:79% accuracy ...

[Anonymous Coward]

It's actually 4;

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And then I notice I have finished my sentence with a semicolon. Am I going crazy?yes:no;