Quantum computing

Cue the qubits

FOR several years, physicists, mathematicians and computer scientists have been worrying at the problem of how to make a quantum computer--a device that would out-perform conventional (or "classical") computers as thoroughly as conventional computers have beaten the abacus. Some think that, because such a machine would rely on those most fragile of phenomena--the quantum uncertainties of the microscopic world--it may never be possible to construct one. Now, as reported at the AAAS by Neil Gershenfeld of the Massachusetts Institute of Technology, quantum computing may have been rescued by a chemists' trick.
     A classical computer is, indeed, little more than a speeded-up abacus. It stores and shuffles binary numbers (those containing only the digits 1 and 0, which it remembers as the "on" and "off" positions of tiny switches, or "bits"). By contrast, the switches in a quantum computer would be far tinier, and thus able (thanks to the strange rules of quantum theory) to be both "on" and "off" simultaneously. Each switch (known in this context as a "qubit") could then, in effect, be doing two calculations at once. Two qubits could thus do four things at once, three qubits could do eight, and so on. A quantum computer with fairly few components could therefore crack vast problems that a classical computer could not solve before the end of time.
     But there is a catch. Anything in such a quantum combination of states is in a highly delicate balance, and any disturbance--including that needed to read the result of a calculation from such a computer--upsets it. It is said to "decohere": to fall completely into one or another of its possible simultaneous states, to the exclusion of the others. This wipes out most of the information it contains.
     Solving the problem of decoherence is thus essential if a quantum computer is ever to be built. It is toward this end that Dr Gershenfeld and his colleague Isaac Chuang, of Los Alamos National Laboratory in New Mexico, believe they have taken a significant step.
     They have done it by abandoning a crucial assumption about quantum devices: that only something tiny, and isolated from almost everything else, could hover in multiple states and so act as a qubit. They worked out that the same effect could be achieved by using the total opposite: a sea of molecules, jostling and tumbling like a band of excitable schoolchildren.
     Molecules contain atoms, and the nuclei of atoms act like tiny magnets. A property called "spin" indicates which way an atomic magnet points. A single nucleus can therefore act as a qubit, its spin pointing perhaps up for "off" and down for "on". A bunch of connected atoms--a molecule--can thus be a quantum computer, capable of as many simultaneous calculations as there are ways of arranging the spins in it. The nuclei, cocooned within the molecule, make far more stable qubits than the other kinds tried so far. And there is, crucially, a way of manipulating the qubits and reading the result of the calculation.
     This tool is nuclear magnetic resonance (NMR) spectroscopy--a technique used by chemists to analyse molecules. It involves putting the molecules in a magnetic field and hitting them with pulses of radio waves, to which they respond with signals that depend on how their spins are arranged.
But these pulses do make the spins decohere. The researchers' answer is to use back-up copies, by having not one molecule, but a small cup containing about a hundred billion trillion of them.
     This was where previous assumptions had to be dropped. If it is so hard to manipulate an isolated qubit or a single molecule without its decohering, then how, when huge numbers of them are colliding with each other, can you tell which ones are doing what, and keep them controlled? It would be like trying to pick out a few children walking due north, and then making sure they kept going in exactly the same direction, in a vast playground jam-packed with kids running around everywhere.
     However, NMR works because in any given bunch of molecules there are slightly more with spins pointing in one direction rather than another. The signals from this small surplus (about one part in a million) stand far enough out from the background noise for the NMR equipment to pick them up. The crucial thing that Dr Chuang and Dr Gershenfeld realised was that the background noise would average out, thus leaving the quantum states of this small surplus of molecules undisturbed. (If the kid going due north is buffeted equally in every direction, he will still end up going due north). So, unlike a single qubit, which is prey to any passing distraction, they could sit in the molecular equivalent of a boiling sea and not lose their calm. These molecules could thus act as multiple copies of a quantum computer. Each radio pulse used to instruct or interrogate them would unbalance a few. But enough would be left acting in concert to last for several dozen computational steps.
     The final hurdle was getting the right answer out at the end. A quantum computation usually gives a variable result which has to be corrected by a final, classical calculation. Lots of quantum computers would produce lots of different answers. However, a quantum computer can do the corrective calculation too; in fact, it turns out, it can do the calculation on itself. Dr Chuang and Dr Gershenfeld managed to apply this technique to all their molecules, so that they gave the correct answer.
     Since writing a paper on the subject (published a few weeks ago in the AAAS's journal, Science), Dr Chuang and Dr Gershenfeld have been putting this into practice. Using the carbon atoms in a molecule called alanine as qubits, they have built a computer that can add one and one and give the result. This may look trivial, but it is way ahead of other approaches. And, by using more complex molecules--the caffeine in a cup of coffee is apparently a candidate--Dr Chuang and Dr Ger shenfeld hope by the end of the year to make a ten-qubit device which could divide the number 15 into its factors. For still larger problems, perhaps cinnamon coffee, in which Seattle abounds, will be suitable.