rose.blog

I was reading a post on Dave Bacon’s blog about a recent quant-ph article. The article questions whether or not fault tolerant quantum error correction can be run on gate model quantum computers, and offers some arguments as to why the answer may be no.

I believe that all of these arguments can (and probably will) individually be refuted. However I think most of the folks eager to begin the rebuttal may be missing the point.

People actually trying to build real QCs have to deal with a large number of real-life problems that are ignored in most theoretical QC. While they may understand that theory has value, they don’t like being told “this is the way things are” by people who have never had to fix a solder joint (yes, there are solder joints in real QCs).

I was involved with a particular case of this recently. A lot of the literature on adiabatic quantum computing (AQC) is wrong, for a very basic physical reason–the omission of realistic environments. Adding a simple thermal environment produces theory that matches with experiment. So when I see a “theorem” about AQC usually it doesn’t apply in practice - ie. it doesn’t match experiment - because the underlying model was not correct.

This experience isn’t unique. A large number of people actually building things take theory with a grain of salt, especially theory originating with a community that may not know what should be included (or not) in the underlying analyses. You may poke fun at Dyakonov because he doesn’t like his physics to be written in Lemmas; I think his point is that if you write physical statements as Lemmas you’re probably a few steps removed from the lab and may not appreciate the subtleties in a real condensed matter physics situation.

The gate model (which Dyakonov attacks) is obviously highly unrealistic for several reasons. Whether FT/QEC can ever be made to work may not even be the worst of these. This lack of realism doesn’t have to matter to theorists because to a theorist how difficult it might be to enable is not their problem. I think if QC theorists really wanted QCs to be built, there would be a lot more communication between these communities as to what is realistic and what isn’t, which would have led to the abandonment of the gate model many years ago.

A previous commenter has shamed me into posting more about QC. So here is a post about QC.

In the TAQC computational model, a quantum computation provides the user with an answer that is exact if the computer is in its lowest energy state at the end of the calculation. If the computer is not in the lowest energy state, the result we get is an approximate answer. The approximation gets worse the higher the energy is.

For this model of quantum computation, errors tend to cause the resulting answer we get to drift away from the exact answer. These errors, which drive the computer into higher energy states, are naturally counteracted by the computer system’s tendency to reduce its energy.

This tendency to reduce energy includes dissipative processes–ones that release energy. These types of process, helpful for TAQC in that they improve the approximation to exact that the computer provides, are anathema to the gate model of quantum computation.

In the gate model, there does not exist the same idea of approximate answers ranked by the energies of the states encoded by a processor. The exact solution to a problem could be any state, even the highest energy state. Errors in this model have to be removed because of this lack of a concept of approximate solution. Any change to the desired application of gates brings the computer into a state which could be very far from the desired state.

This unfortunate situation, that the gate model requires error correction in order to work at all, has led to a commonly repeated and very misleading QC dogma. Here it is in a recent preprint from Laforest et.al. quant-ph:0610038:

QEC [quantum error correction] is a requirement for scalable quantum information processing (QIP)

While technically this is true, what the authors are implying is that active quantum error correction, which is a complex and ridiculous process that will probably never be used outside a research lab ever, is required for quantum computation. It isn’t, which is a good thing. In the TAQC model, quantum error correction occurs passively; nature itself shepherds the system towards better and better approximate answers.

I would even go as far as to bet that no QC that requires active error correction will EVER perform a computation beyond the scope of conventional systems; having architectures and systems that are passively protected against errors is Rule #1 for anyone serious about building large-scale quantum computers.

Here is a link to Dawkins’ new foundation. Here is a quote from a presentation on the site:

The enlightenment is under threat. So is reason. So is truth. So is science, especially in the schools of America. I am one of those scientists who feels that it is no longer enough just to get on and do science. We have to devote a significant proportion of our time and resources to defending it from deliberate attack from organized ignorance. We even have to go out on the attack ourselves, for the sake of reason and sanity. But it must be a positive attack, for science and reason have so much to give. They are not just useful, they enrich our lives in the same kind of way as the arts do. Promoting science as poetry was one of the things that Carl Sagan did so well, and I aspire to continue his tradition.

Somebody asked me who Richard Dawkins was after my last post, uhhh just the most brilliant communicator of science of all time dumbass!!! Anyway my vote for Greatest Book Ever Written is Richard Dawkins‘ The Ancestor’s Tale, which I think should be required reading for all living beings. Go buy it and read it.

Of course you can’t go wrong reading anything else he’s ever written–some of the more well-known books like The Selfish Gene, The Blind Watchmaker and The God Delusion are also superb.

Gives me hope that we’re not all doomed to an existence defined by Fox News, George W. Bush AKA the King of Jesusland and that retard from Iran (who the not-quite-but-almost-as-retarded Glenn Beck calls President Tom, which is funny on several levels) …

I noticed today that David Deutsch has some lectures posted on QM/QC, and since he’s the closest thing to Richard Dawkins that QM/QC has, I’ve provided a link to them here and in the widget bar. Beautiful stuff…definitely worth taking a look at it you’re interested in what QM really tells us about the world.

I’ve added another category to the widget bar, linking to some presentations about (mostly) quantum computing related stuff.