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Due to the inherent vagueness of the quantum world, a quantum
computer can perform numerous sums at the same time. Last year, physicists
thought this was proved in the laboratory, but there is bad news.
Although the sums were modest, there was great euphoria when American researchers under direction of Isaac Chuang of IBM reported a breakthrough in so-called quantum computing. In a laboratory full of nuclear spin magnets, radio transmitters and test tubes they let the molecules in one thimble of chloroform determine in one step which of the numbers 1, 2, 3 and 4 are odd and greater than 2. Three, said the fluid; the champagne popped and the authors published extensively in prestigious journals. Every possible algorithm on an ordinary digital computer would have needed at least two steps. That bottle was slightly premature, says Sam Braunstein of the University of Wales in Bangor. Together with like-minded Britons he published a complicated article in Physical Review Letters last week (2 August), which basically argues that it was shear luck that the Americans found the right answer in their chloroform. It doesn't need to have been quantum computation. That is bad news, because since a few years, computation using magnetic fields and chloroform is regarded as the most practical way to implement quantum computation. Chuang's tests of last year were considered a milestone, and his papers are already widely cited. Wrongly, as it now appears. Quantum computation uses the inherent vagueness of the quantum world. In that world, particles or molecules can exist in two or more states simultaneously. Only after a measurement they obtain one identity. By manipulating the particles, all the states can be addressed at the same time. Take every state as a computer word of a number of zeros and ones (a byte), and a molecule becomes a computer which can treat numerous bytes simultaneously, rather than one-by-one as in an ordinary digital computer. That quantum mechanical disunity is exactly the promise which drives the research on quantum computers. What takes ages with conventional computers can theoretically be done on a quantum computer within reasonable time. When Peter Shor from AT&T first showed this in 1994, banks and security services were immediately interested. Unbreakable security codes could then be cracked by a quantum computer. However, five years on, this is still a future prospect. Chloroform CHCl_3 consists of a hydrogen atom H and a carbon atom C, which both can have their intrinsic magnetic field (spin) in two directions: up (1) and down (0). Every chloroform molecule can therefore take on four quantum states (00,11,10,01). These states can be manipulated with radio waves. For example by `flipping' the states from 0 to 1 and vice versa the logical operation `NOT' is applied to all four states. However, this is easier said than done. Quantum effects only emerge when during the operation the molecule is isolated from its environment. The probability for this is vanishingly small, but sometimes a molecule slips through untouched. The art of conducting experiments like Chuang's is to detect the weak signals from exactly those molecules. The idea is in principle correct, says Sam Braunstein, although in no current chloroform experiment such a quantum state is produced with certainty. At least in the `chaotic' mixture of touched and untouched molecules we cannot determine the origin of a signal if a molecule has less than 14 variable spins. This is, however, not the whole story, say Lieven Vandersypen, a Belgian physicist who collaborated with Chuang via Stanford University. `We did not realize this, but our algorithm still really works faster than any conventional computation.' Critic Braunstein also thinks that his comments do not necessarily mean the end of quantum computation with nuclear spins in molecules. `On the contrary. Apparently there are other, simpler ways to perform quantum computation than we have so far assumed' he says. And chloroform computationer Vandersypen agrees whole-heartedly: `there is still much to learn.' Understood or not, in his group experiments are now conducted with much larger molecules capable of accessing many more quantum states than the four of chloroform. The computational power then grows rapidly. And are those molecules equipped with 14 spins? Vandersypen, well instructed by his ambitious boss Chuang, leaves this aside. The competition just has to wait for their next publication. Martijn van Calmthout De Volkskrant, 14 August 1999 Translated by Pieter Kok |