Wednesday, February 14, 2007

World’s First Commercial Quantum Computer Demonstrated

I did a degree in maths & programming in the eighties and had a fabulous time - however, one of the subjects that came up often (particularly when doing cryptography) was that mythical beast the quantum computer - tutors would say things like ¨ you can see all possible solutions are computable at once¨ - which always made me ask how you'd separate the correct answer from all possible answers. In terms of crypto it seemed like you were just re-iterating the size of the key-space. Anyhow - D-Wave Systems of Canada have launched a processor that uses superconduction to contain the quantum elements - that's one of the problems - how do you shield the quantum processes from the 'noise' of the universe? Traditionally research has focused on using laser interference to create quantum states but the accuracy of such rigs makes them impractical.
Anyhow - the article on Wikipedia got me thinking;
...the contents of the qubit registers can be thought of as an 8-dimensional complex vector. An algorithm for a quantum computer must initialize this vector in some specified form (dependent on the design of the quantum computer). In each step of the algorithm, that vector is modified by multiplying it by a unitary matrix. The matrix is determined by the physics of the device. The unitary character of the matrix ensures the matrix is invertible (so each step is reversible).

So I think I understand that to mean that the array contains complex values with both size and direction - but in eight dimensions - wow! The unitary matrix which is the second parameter in whatever function is used (¨determined by the physics of the device¨) and the function is two-way.
Upon termination of the algorithm, the 8-dimensional complex vector stored in the register must be somehow read off from the qubit register by a quantum measurement. However, by the laws of quantum mechanics, that measurement will yield a random 3 bit string (and it will destroy the stored state as well). This random string can be used in computing the value of a function because (by design) the probability distribution of the measured output bitstring is skewed in favor of the correct value of the function. By repeated runs of the quantum computer and measurement of the output, the correct value can be determined, to a high probability, by majority polling of the outputs. In brief, quantum computations are probabilistic

That seems almost (but not quite) as much of a cop-out as my undergraduate lecturer!
I suppose that if you took a guy doing my job from fifty years ago he'd have trouble understanding not only the technology we now use (digital over analogue, compressed video, digital control systems over servos etc.) but he'd be hard pushed to see why we'd want to do things the way we do. I think I'll feel the same way about quantum computers when they eventually make it to prime time.

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