From The Economist
Quantum computing: Not a leap, perhaps, but two important steps on the way to making quantum computing practical
Some technologies seem a long time coming. Their story has a familiar script: a breakthrough is hailed; overblown expectations are aroused; disappointment follows. Away from the public’s impatient gaze, however, tinkering scientists turn out incremental improvements, furtively catching up with the revolutionary rhetoric of yore. This appears to have been the case with the sequencing of the human genome. It also illustrates the recent history of quantum computing.
An ordinary computer stores and processes information using bits, which take the value of either one or zero (physically represented by different voltages of electric current). In the bizarre world of quantum mechanics, however, subatomic particles can exist in several states at once. Such “superposition” means, for instance, that the property of an electron known as its spin can be not only “up” (representing, say, one) or “down” (representing zero) but also some combination of the two. In quantum computing, such superposed values are named qubits.
Additional qubits can be added by a process called entanglement. Each extra entangled qubit doubles the number of parallel operations that can be carried out. A three-qubit device could run eight operations, a four-qubit system 16, and so on. At least in theory. In practice the results so far have been disappointing. Experimental quantum computers require exotic materials and work only at a tad above absolute zero. But the latest incremental improvement may help lift the field out of the slough of despond. First, it does its computing in silicon—the workhorse of more familiar computers. Second, it does it at the balmy temperature of -269˚C, or four degrees above absolute zero.
The magicians who have pulled off this double trick are Thornton Greenland of University College, London, and his colleagues. As they describe in this week’s Nature, they first doped the silicon with phosphorus atoms. They then used a short, high-intensity pulse from a fancy laser to jolt electrons orbiting those phosphorus atoms into what is known as a Rydberg state.
In this state, each atom’s quantum wave-function—its effective size—extends several billionths of a metre from its nucleus. That is huge in atomic terms, and permits the entanglement of atoms that would normally be too far apart to interact. This, in turn, means they can become collaborative qubits.
Once their wave-functions had been expanded, the atoms were zapped again, inducing them to emit a controlled burst of light called a photon echo. This would be used to read out the result of any calculation the system had performed.
Magical mystery tour
The result is something that could, in principle, do calculations and spit out the answers—and which could be made industrially using the sort of equipment that turns out conventional chips. That it works at four degrees above absolute zero is a bonus. It might sound pretty chilly, but it is a temperature that can be maintained fairly easily using liquid helium, which boils at 4.22 degrees above absolute zero.
Dr Greenland’s discovery, then, brings practical quantum calculation closer. And a second paper in Nature, by Morgan Hedges, of the Australian National University in Canberra, and his colleagues, has advanced another aspect of the field by describing a type of quantum memory that uses light.
Atoms and electrons are not the only particles that can get entangled. Photons, the particles of light, are also susceptible. If they could be integrated into the world of quantum computing, that would help with the transfer of qubits from place to place. The problem with light, though, is its speed. However handy its high velocity is for whizzing information over great distances through optical fibres, it is unhelpful if any calculating has to be done. For that to happen light needs to be pinned down.
In 2008 Hugues de Riedmatten of the University of Geneva and his team managed to do that. They built what was, in essence, an echo chamber on a chip and held a single photon there for a microsecond (a period when that photon would normally have expected to cover about 300 metres).
Dr Hedges’s echo chamber did considerably better than that. By using such exotic materials as yttrium orthosilicate crystals and praseodymium ions it trapped photons in bunches of as many as 500 at a time. It was also good at preserving entanglement, increasing the amount of quantum information it could hold without being corrupted or destroyed to a level that might be practically useful.
Sadly, the developments of Dr Greenland and Dr Hedges do not spell supermarket shelves stacked with quantum PCs any time soon. They do mean, though, that the tinkerers have more to tinker with—and that, ultimately, is how new technologies really develop.