The personal computer on my desktop is hundreds of times as fast, and has thousands of times as much memory, as the mainframe computer that served my entire university when I was a student. Such advances in the processing speed and storage capacity of computers are expected to continue until the laws of physics impose certain limits. After all, we cannot shrink atoms or increase the speed of light.

Now, imagine a computer technology that eventually approaches these limits. Imagine further that every star in every galaxy in the observable universe could somehow be fashioned into computers of this ``ultimate'' type. That would be a lot of very fast computers. Or if they were connected together we could think of them as a single, massively parallel computer: call it the ``Universe Computer.''

Admittedly, there are tasks - word processing, for example - at which this imaginary computer would be no more useful than any one of its constituent computers operating independently. But for large, repetitive trial-and-error tasks, such as code breaking, the speed and power of this integrated ``Universe Computer'' would be vastly superior to anything we could ever hope to build, right?

Hold that thought.

Quantum theory - the branch of physics that deals with elementary particles and the microscopic properties of matter - has produced some of our deepest insights into nature, and describes some startlingly counter-intuitive phenomena. For example, it implies that elementary particles, rather than being located at one position at a time, travel on several trajectories simultaneously.

No one disputes that quantum phenomena, if they could be harnessed, would revolutionize information processing, enabling ways of computing that no existing computer, even in principle, would be capable of duplicating. Among the tasks for which quantum computing would be ideally suited are ``algorithmic searches.'' Put simply, algorithmic searches are what computer programmers use as a last resort when looking for a mathematical needle in a haystack: they make the computer try every possible answer in turn until it finds the right one.

Obviously, the resources required for such searches are proportional to the number of possible answers: common sense tells us that trying a thousand possibilities requires a thousand times as many operations as trying one. Trying a million possibilities requires a million times as many operations.

But our ordinary common sense does not apply in fundamental physics. In 1996, the computer scientist Lov Grover discovered a quantum algorithm - a way to program a quantum computer - that could try out a million possibilities in only a thousand times the time needed to try one, and a trillion possibilities in only a million times the time of one, and so on, without limit.

What would happen inside a quantum computer when it performs an algorithmic search? The unseemly truth is that most physicists are perplexed and embarrassed by this question. Many explain away quantum phenomena with weasel words, or worse, they simply eschew explanation altogether. True, quantum phenomena cannot be observed directly. But we can deduce their existence and attributes by measuring their effects on things that
*are
* directly observable. We have never observed live dinosaurs, either, but we know they existed - and quite a lot about how they worked - from the fossil record.

A growing minority of physicists, myself included, accept the ``many universes'' interpretation of quantum mechanics. We concluded that what we observe as a single particle is really one of countless similar entities in different universes, subtly affecting each other through a process called ``quantum interference.'' To us, no mystery exists in quantum computation, only wonder.

Quantum computing, according to this view, is possible because a quantum computer performs vast numbers of separate computations in different universes and then shares the results through quantum interference. The equations of quantum theory describe this phenomenon step by step. But because such information sharing is achievable
*only
* through quantum interference, these same equations also drastically limit the types of task that quantum computation should be able to perform, or speed up. Direct communication between universes is, for example, ruled out.

In fact, only a handful of potentially useful quantum algorithms are known at present. Grover's algorithm is one. Other known quantum algorithms will easily be able to crack the most widely used secure cryptographic systems of today. Coincidentally, cryptographic systems which themselves use quantum computation are already commonplace in laboratories, heralding the development of communication that is both perfectly secure - even against quantum attack - and immune to future advances in mathematics or technology.

Quantum cryptography happens to be relatively easy to implement. Unfortunately, we have no computers powerful enough to run any other useful quantum algorithm; building powerful quantum computers is a major scientific and technological challenge for the coming decades. But theoretical physicists already know how many different types of components are required to make a quantum computer, and how complicated these components must be. The astonishing answer is that virtually any interaction between two information-carrying entities, including atoms or elementary particles, will do. As the physicist Seth Lloyd commented, ``almost anything becomes a computer if you shine the right kind of light on it.''

It has long been assumed that a single type of machine, given time and memory, could simulate the behavior of any other state of matter. It turns out that existing computers, or even the imaginary ``Universe Computer,'' are not the ones. But a general-purpose quantum computer would be. In quantum physics, this ``computational universality'' is part of the essence of all matter - and thus of the comprehensibility of nature. No other branch of physics involves such wide-ranging interaction between theory, experiment, technology, and philosophy. No other field of scientific research holds more promising implications for our understanding of the universe.

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