At the solstice: Shining light on quantum computers

December 21, 2012, University of Queensland

At the solstice: shining light on quantum computers
The University of Queensland's Dr Matthew Broome peering over the BosonSampling quantum computer.
(—An Australian-American team has shone light—literally—onto the question of whether quantum computers are actually more powerful than conventional counterparts.

"Famously, quantum computers promise a more efficient means of computation, for example using a technique known as `fast factoring' to efficiently crack encryption codes that form the basis of today's internet security," said the study's lead author, Dr Matthew Broome, of the University of Queensland.

Surprisingly it's still not known whether quantum computers are the only way to do this efficiently, or whether conventional computers can solve the problem almost as quickly.

In a paper in Science this week, scientists from The University of Queensland and the Massachusetts Institute of Technology (MIT) described the first experimental steps towards answering this question, building a so-called 'BosonSampling' device.

The device implemented a form of quantum computation where a handful of single photons were sent through a photonic network.

The team then sampled how often the photons exited the network.

"Although this sounds simple, for large devices and many photons, it becomes extremely difficult to predict the outcomes using a conventional computer, whereas our measurements remain straightforward to do," said Dr Broome.

Testing this device—proposed in late 2010 by co-author Associate-Professor Scott Aaronson, and his colleague Dr Alex Arkhipov, both from MIT—will provide strong evidence that quantum computers do indeed have an exponential advantage over conventional computers.

The experimental team leader at UQ Professor Andrew White said: "Scott and Alex's proposal was a 94-page mathematical tour-de-force."

"We genuinely didn't know if it would implement nicely in the lab, where we have to worry about real-world effects like lossy circuits, and imperfect single and detectors."

Confirming that the BosonSampling device behaves as expected paves the way for larger and larger instances of this experiment.

The prediction is that with just tens of photons it can outperform any of today's supercomputers.

"I am excited to see that the first proof-of-principle demonstrations of BosonSampling have been shown—even if only with 3 , rather than the 30 or so required to outperform a classical computer," said Associate-Professor Aaronson.

"I did not expect this to happen so quickly."

Related experimental work was published in the same issue of Science by J. Spring et al.

Read: Boson samplers offering promise for new kinds of computing devices

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not rated yet Dec 23, 2012
At the moment, when the computational power of classical computers becomes limited with uncertainty principle, then the quantum computers cannot bring any new progress into it. The problem of quantum computers is, they're fast, but very approximate. At the moment, when you repeat the same calculation multiple-times for to achieve the same level of precision/reliability like the classical 64-bit computer, then every advantage of quantum computer will simply evaporate. IMO it's just a waste of tax payers money, which could be used for development of cold fusion, for example - i.e. way more effectively.

And if you're impressed with the possibility, you could do the mathematical operations with multiple qbits at the same moment, then you should be warned, that the stability of these entangled qabits decreases with the number of entangled states geometrically.

Memo: the physical laws are binding for everyone.
not rated yet Dec 28, 2012
Quantum computing is better than classical only on some problems, like fourier transform or factoring.

Sure quantum computing only gives a very probable result on those problems. But you can just verify if that result was the correct one by then doing (classically or not) the problem in the reverse order (finding the question from the answer).

Still, even if probabilities are involved, it is very highly probable that the answer will be found a lot more quickly than when using a classical algorithm.

The real power of quantum computing is that a qbit can be in a superposition of 0 and 1 at the same time. Incertitude of position or momentum of the particles is another subject which is totally independant of the existance of superposition.

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