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Blaine Bateman

Analog quantum computing

Blaine Bateman
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1/6/2016 3:28:59 PM
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Thanks for the interesting thoughts on noise and accuracy. There are, as I understand it techniques exist to eliminate errors in digital processing. That works well for distinguishing 1s from 0s. But as you point out, that may not work or even make sense in quantum or analog computing. Both Google and IBM say they have figured out quantum error correction. I must admit I don't understand how that works.

Katie OQ
Katie OQ
1/6/2016 3:11:16 PM
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This op ed piece is commendable for providing many very interesting references to recent work in the broad - and exponentially widening - field of quantum computing. I confess to being quite unable to keep abreast of these developments. However, as for the suggested quasi-equivalence of quantum and analog computing, I feel this might be more than a bit misleading as to the important ways in which these differ.

Nevertheless, there are some intriguing similarities. Consider, for example, a fully-analog circuit of almost any complexity, existing as, let's say, a breadboard. [Young folks: this is a circuit built up from a selection of primitive elements place on the "board" – sans bread – and wired by hand. Primitives range from such basic elements as R, L and C, through discrete transistors, up to analog ICs]. Let's omit for now any energy-storage elements – principally L's and C's, but there are, of course, others – having significant effect on the circuit's time-domain behaviour. One could refine all these definitions.

But here's the similarity: When the primary source of [DC] power is applied, the circuit instantly "solves" the equations for all the elements, both individually and also as an interactive ensemble. It does not need to be "told" about the physics of these elements, using modeling equations, because the circuit is itself a piece of the real, physical world. In this respect, it is an "analog" of nothing. Likewise, the circuit's response to any other stimuli applied to it will likewise occur without any recourse to some sort of computation. In this respect it IS its "own analog computer".

Now, we may question the absolute accuracy of any "result' – say an output voltage of a simple amplifier – of this tangle. Whatever solution it happens to settle on – by the rapid conversation that occurs when any stimulus is altered – is its own "private" solution. The matter of providing accuracy never occurs to an analog circuit. It might be quite accurate – enough for the ensemble to be used as a component in a useful [that is, high-precision] Analog Computer; but, as far as the circuit is concerned, it is simply solving its internal equations, whose accuracy in some absolute sense can range from poor to excellent.

We may yet find another similarity here to quantum computing. While small ensemble of quantum elements – a few qubitsworth - may fortuitously appear to behave quite well, and very, very fast in operation, I have a hunch that as they become larger in extent – to employing hundreds of elements – they will be prone to [at least one of] the same enemies as analog computing, namely, universal noise. Operation at low temperatures may squelch basic qunoise; but some types of circuit error - those attributable to shot noise for example, which is not temperature dependent [at least, as generally explained and modelled, although, as an aside, there may be some errors of rigour here, as far as real devices are concerned], and even some fancy variety of 1/f noise - may eventually show up in large quantum computers yet to be designed and built.


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