## David Deutsch and quantum constructor theory

**from the putting-the-multiverse-to-work dept.**

David Coutts writes *"David Deutsch, founder of the quantum computer field, believes that quantum computers will work by drawing upon the processing power of the multiverse. His book "The Fabric Of Reality" is a great read. Deutsch is working on a theoretical framework to prove whether or not quantum computers can be built. He calls this the 'quantum constructor theory'."* Deutsch also hopes that his theory would answer questions about nanotechnology. In this interview, "It's a much bigger thing than it looks," posted last November on the Third Culture section of The Edge, he explains some of his ideas.

*…"a big technological question in my field at the moment is, can useful quantum computers actually be built? The basic laws of physics seem to permit them. We can design them in theory. We know what physical operations they would have to perform. But there is still room for doubt about whether one can build them out of actual atoms and make them work in a useful way. Some people are still pessimistic about that, but either way, that debate is not really a scientific one at the moment, because there is no scientific theory about what can and can't be built. Similar questions are raised by the whole range of nanotechnology that has been proposed in principle. So that's where a quantum constructor theory is needed."*

February 11th, 2001 at 11:40 AM

Quantum ComputersCan anyone clear up some basics with quantum computers? This seems to be a term that is used to descibe different things in the media, ranging from DNA based computing to quantum dots as data storage to utilizing the spin of electrons, To utilizing the multiverse and parallel universes. Which of these would be an accurate description of quantum computing?

February 11th, 2001 at 9:26 PM

Re:Quantum ComputersI believe that David Deutsch is one of the founders (if not THE founder) of quantum computers, so perhaps his book "The Fabric Of Reality" is a good place to start. I really enjoyed it, though (as the name suggests) it's about a bigger subject than quantum computers. DNA computers are, I thought, a separate field. However, I take your point about quantum dots and spin states (as opposed to using the multiverse). It IS confusing! Deutsch is quite adamant about the multiverse interpretation (as opposed to the Copenhagen interpretation) of the famous Shrodinger's cat experiment. It's quite a facsinating thought. In his book, Deutsch challenges people to state how quantum computers will work if they DO NOT draw upon the multiverse. Regards, David

February 12th, 2001 at 11:38 AM

quantum computing oversimplifiedSome people might use the term "quantum computing" to refer to any use of effects considered "quantum mechanical" rather than "classical," but nearly all of the interest today is in exploiting "quantum parallelism", or what Deutsch interprets in terms of "the multiverse".

This "parallelism" can be understood as an extension of the Feynman path integral approach to quantum mechanics, in which the probablity of a physical system evolving from state A to state B is computed by summing over all possible paths that lead from A to B and weighting each path with a phase factor derived from the action integral of the path. See vol. 3 of the Feynman Lectures on Physics for the classic introduction to this idea.

In quantum computing, the computer evolves along all possible paths from its initial state, and the probability of any particular final state will be given by a sum of all paths that lead to that state. Another way of describing this is to say that the computer evolves along an exponentially growing (multiplying with each step) number of paths, and in the final step all these parallel computations interfere with each other to determine the probabilities of various outcomes.

Ideally, you want to determine a particular outcome with unit probability. That's because there is really only one computer and when you measure the final state, you will only get one answer. "Quantum parallelism" is therefore not the same as ordinary parallel computing, where you can get, say, an entire table of numbers out at the end of one computation, or perhaps the solution to a partial differential equation describing an airflow. In quantum computing you only get one number at the end, but that number represents an exponentially large number of paths of computation, and if you have been clever enough to find an algorithm that interferes all the paths in the right way, the answer you get can represent an integration of all that exponentially large amount of computing.

Unfortunately, to date only two potentially useful algorithms have been found, and only one of them (the Shor factoring algorithm) really takes advantage of the exponential power of quantum computing.

David Deutsch's most important contribution was to formulate the idea of a Turing machine-like general quantum computer exploiting quantum parallelism. But the idea of quantum computing represents the work of many people, including Paul Benioff, who investigated the possibility of classical Turing machines using quantum mechanical evolution in the early 1980s, Feynman, who suggested the possible relationship between QC and nanotechnology as early as 1959, and who also pointed out the fact that quantum computing is potentially more powerful than classical computing, since classical computers cannot simulate quantum mechanics efficiently, while quantum computers should be able to. Deutsch's 1985 paper on the universal quantum computer was important, but it was really Peter Shor's 1994 discovery of the factoring algorithm, and subsequent work by Shor and others on quantum error correction, that led to the explosion of interest in the field.

In his book, Deutsch insists that many-worlds is needed in order to make it possible to "understand" how quantum computing is possible. But this is rather silly. How well do we "understand" other aspects of physics, even simple things like classical motion and gravitation? Deutsch's demand for "explanation" is reminiscent of religious people who ask, "If you don't believe in God, then how did everything get here?" My answer is simple: I don't know. What made you think you would ever find out? Maybe we will, maybe we won't. We should be grateful that at least we know how to grow food and build houses to keep the rain off our heads. Quantum mechanics predicts that quantum computing should be possible, regardless of whether you interpret it in terms of a multiverse or not. Insisting that many-worlds must be true because you think it makes the facts of quantum mechanics easy to understand, even though it does not modify or predict any observable fact, assumes that someone promised you that "the fabric of reality" would be easy to understand, when, obviously, it isn't.

April 4th, 2008 at 11:25 AM

If no one ever came up with a theory that there is a god, then no one would seek to prove or disprove that idea. Any explanation is better than no explanation for it advances scientific research. This is the point that David Deutsch makes in his book. He also makes a very logical and supported argument for supporting the theory of the multiverse. The reason that he asks scientists for an explanation is so that he can seek to disprove their theory! to give a theory without an explanation is much more like religious behavior.

Thank you