A COMPUTER proceeds one step at a time. At any particular moment, each of its bits—the binary digits it adds and subtracts to arrive at its conclusions—has a single, definite value: zero or one. At that moment the machine is in just one state, a particular mixture of zeros and ones. It can therefore perform only one calculation next. This puts a limit on its power. To increase that power, you have to make it work faster. But bits do not exist in the abstract. Each depends for its reality on the physical state of part of the computer’s processor or memory. And physical states, at the quantum level, are not as clear-cut as classical physics pretends. That leaves engineers a bit of wriggle room. By exploiting certain quantum effects they can create bits, known as qubits, that do not have a definite value, thus overcoming classical computing’s limits. Around the world, small bands of such engineers have been working on this approach for decades. Using two particular quantum phenomena, called superposition and entanglement, they have created qubits and linked them together to make prototype machines that exist in many states simultaneously. Such quantum computers do not require an increase in speed for their power to increase. In principle, this could allow them to become far more powerful than any classical machine—and it now looks as if principle will soon be turned into practice. Big firms, such as Google, Hewlett-Packard, IBM and Microsoft, are looking at how quantum computers might be commercialised. The world of quantum computation is almost here. A Shor thingAs with a classical bit, the term qubit is used, slightly confusingly, to refer both to the mathematical value recorded and the element of the computer doing the recording. Quantum uncertainty means that, until it is examined, the value of a qubit can be described only in terms of probability. Its possible states, zero and one, are, in the jargon, superposed—meaning that to some degree the qubit is in one of these states, and to some degree it is in the other. Those superposed probabilities can, moreover, rise and fall with time. The other pertinent phenomenon, entanglement, is caused because qubits can, if set up carefully so that energy flows between them unimpeded, mix their probabilities with one another. Achieving this is tricky. The process of entanglement is easily disrupted by such things as heat-induced vibration. As a result, some quantum computers have to work at temperatures close to absolute zero. If entanglement can be achieved, though, the result is a device that, at a given instant, is in all of the possible states permitted by its qubits’ probability mixtures. Entanglement also means that to operate on any one of the entangled qubits is to operate on all of them. It is these two things which give quantum computers their power. Harnessing that power is, nevertheless, hard. Quantum computers require special algorithms to exploit their special characteristics. Such algorithms break problems into parts that, as they are run through the ensemble of qubits, sum up the various probabilities of each qubit’s value to arrive at the most likely answer. One example—Shor’s algorithm, invented by Peter Shor of the Massachusetts Institute of Technology—can factorise any non-prime number. Factorising large numbers stumps classical computers and, since most modern cryptography relies on such factorisations being difficult, there are a lot of worried security experts out there. Cryptography, however, is only the beginning. Each of the firms looking at quantum computers has teams of mathematicians searching for other things that lend themselves to quantum analysis, and crafting algorithms to carry them out. Top of the list is simulating physics accurately at the atomic level. Such simulation could speed up the development of drugs, and also improve important bits of industrial chemistry, such as the energy-greedy Haber process by which ammonia is synthesised for use in much of the world’s fertiliser. Better understanding of atoms might lead, too, to better ways of desalinating seawater or sucking carbon dioxide from the atmosphere in order to curb climate change. It may even result in a better understanding of superconductivity, permitting the invention of a superconductor that works at room temperature. That would allow electricity to be transported without losses. Quantum computers are not better than classical ones at everything. They will not, for example, download web pages any faster or improve the graphics of computer games. But they would be able to handle problems of image and speech recognition, and real-time language translation. They should also be well suited to the challenges of the big-data era, neatly extracting wisdom from the screeds of messy information generated by sensors, medical records and stockmarkets. For the firm that makes one, riches await. |

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