Anyone old enough to remember the Sinclair ZX-81 computer may have the merest inkling of what those four bits may be worth by the year 2020. The writer's series 'Quantum' is to be broadcast on Radio 4, beginning at 9pm on Wednesday, 1 December. When cooled to very low temperatures, and tweaked in the right way, an array of 300 of these things, working together as qbits, could well form the basis of the ultimate quantum computer. And Clark, unlike Deutsch, has his feet firmly on the ground when it comes to justifying such research: "because," he says, "new technology creates wealth". That does not necessarily mean personal wealth, if you work at a British university, but economic growth. Clark emphasises that his group is not the only one with radical new ideas about how to make quantum machines, and says that together all the researchers working in this field have now "laid the basis for a whole new technology, that will clearly involve quantum computers". And once you've got those super-powerful quantum computers, you can use them to push the technology further - certainly to help design a working teleportation system for inanimate objects, and possibly (according to a few theorists) opening up the prospect of some form of time travel.

Terry Clark and his colleagues have pioneered the development of devices that are big enough to see, and hold in your hand, but which behave like single quantum entities. These superconducting quantum interference devices (or Squids) are each about the size of a wedding-ring, with a constriction at one point in the ring. If we ever could build a computer that behaved as if it contained more atoms than there are in the entire Universe, how else could you explain what is going on except by saying that there are other, parallel universes out there, which are somehow connected to our own by quantum phenomena? To Deutsch, a theorist par excellence, the possibility of showing the many worlds idea to be correct is as good a reason as any to build the ultimate quantum computer. Given the pace of progress with conventional computers, who is to say that all this will not happen within a human lifetime? But it won't happen using tiny things such as atoms as the qbits, just as modern computers no longer use the glass valves on which the first machines were based. But Deutsch says that there are already plans for a quantum computer using 32 qbits (4 qbytes); and Paul Davies, of the University of Adelaide, calculates that a 300-qbit quantum computer would (thanks to the power of exponentials) be more powerful than what he calls "a conventional clunk clunk" computer that used every atom in the Universe as its on-off switches. This raises an interesting point about the nature of quantum reality, which Deutsch is quick to point out. In this case, if 100 alternative "yous" build identical 10-qbit computers, each reality gets access to a thousand-bit computer. That's still some way short of the power of the machine we use daily, and it is tricky trying to make machines using entities such as individual atoms and photons as qbits. So, crudely speaking, a quantum computer containing just 10 qbits would be as powerful as a conventional machine containing 1,000 bits (a kilobit, one-eighth of a kilobyte). But the most amazing thing is that if you feed a problem into such a quantum computer, it behaves as if all the calculations are carried out on this "supercomputer", then gives you the answer on your simple little machine. Where does all this computing power come from? It looks as if you have got something for nothing - or, at least, 1,000 bits for the cost of 10.

But David Deutsch, a leading proponent of the "many worlds" interpretation, points out that there are many copies of the computer being built by many copies of you in adjacent universes. So a machine with six qbits working together isn't three times as powerful as a machine with two qbits, but eight times as powerful. It may not sound all that dramatic But exponentials quickly run away with you. Whereas with a conventional computer, if you double the number of bits you essentially double the power of the computer, if you double the number of qbits in a quantum computer you square its power, tripling the number of qbits cubes its power, and so on. If you have a string of qbits (even a string as short as four), this means that in some sense they exist in every possible "on-off" combination at once - as if they are carrying out every possible calculation simultaneously. Physicists still argue about what this means - whether it is just some fuzzy quantum superposition (easier to believe for atoms than for cats!), or whether it means there is an entire Universe for every possible combination of "on" and "off" switching in the string of qbits Or maybe both "explanations" are wrong Remember, "nobody understands quantum physics" But there is no doubt about the practical implications. A variation on the theme says that when faced with such a quantum choice, the entire Universe splits into two realities, one with a dead cat and one with a live cat. Now apply that to quantum bits.