Quantum effects are vital to modern electronics. They can also be a damnable nuisance. Make a transistor too small, for example, and electrons within it can simply vanish from one place and reappear in another because their location is quantumly indeterminate. Currents thus leak away, and signals are degraded.
Other people, though, see opportunity instead. Some of the weird things that go on at the quantum scale afford the possibility of doing computing in a new and faster way, and of sending messages that — in theory at least — cannot be intercepted. Several groups of such enthusiasts hope to build quantum computers capable of solving some of the problems which stump today’s machines, such as finding prime factors of numbers with hundreds of digits or trawling through large databases. They gave a progress report to the annual meeting of the American Association for the Advancement of Science (AAAS) in Vancouver.
At the core of their efforts lie the quantum-mechanical phenomena of superposition and entanglement. An ordinary digital computer manipulates information in the form of bits, which take the value of either 0 or 1. These are represented within the computer as different voltages of electric current, itself the result of the electron’s charge. This charge is a fixed feature of all electrons; each has the same amount of it as any other. But electrons possess other, less rigid properties like spin, which can be either “up”, “down” or a fuzzy, imprecisely defined combination of the two. Such combinations, known as superpositions, can be used to construct a quantum analogue of the traditional bit — the qubit.
Entanglement, meanwhile, is the roping together of particles in order to add more qubits. Each extra qubit in a quantum machine doubles the number of simultaneous operations it can perform. It is this which gives quantum computing its power. Two entangled qubits permit four operations; three permit eight; and so on. A 300-qubit computer could perform more concurrent operations than there are atoms in the visible universe.
Unfortunately, such a machine is not in the offing. Entanglement and superposition are delicate things. Even the slightest disturbance causes qubits to “decohere”, shedding their magical properties. To build a working quantum computer, qubits will have to become more resilient, and progress so far has been slow.
The first quantum computations were done in the lab in 1995. Since then various teams have managed to entangle as many as 14 qubits. The record holders, a group in Innsbruck, use a device called an ion trap in which each qubit exists as a superposition of a rubidium atom at different energies. Raymond Laflamme and his colleagues at the University of Waterloo, in Canada, have managed to entangle 12 qubits by performing a similar trick, entangling certain atoms within a single molecule of an amino acid called histidine, the properties of which make it particularly suited to such experiments.
The problem with these approaches is that they will not be easy to scale up. Ion traps reside inside big vacuum chambers, which cannot easily be shrunk. And a molecule of histidine contains only so many suitable atoms. So the search is on for more practical qubits.
One promising approach is to etch qubits in semiconductors. Charles Marcus, previously of Harvard University and now at the University of Copenhagen, has been using electrons’ spins to do this. Single-electron qubits decohere quickly, so his team decided instead to create a qubit out of two electrons, which they trapped in “quantum dots”, tiny semiconducting crystals (of gallium arsenide, in this case). When two such dots are close together, it is possible to get an electron trapped in one to pop over and join its neighbour in the other. The superposition of the two electrons’ spins produces the qubit.
Marcus’s team have so far managed to stitch four such qubits together. An array of clever tricks has extended their life to about 10 microseconds — enough to perform the simple algebraic operations that are the lifeblood of computing. They hope to extend their life further by using silicon or carbon, the atomic nuclei of which interfere less with the entangled electrons than do those of gallium arsenide.
John Martinis and his colleagues at the University of California, Santa Barbara (UCSB), meanwhile, have been trying to forge qubits from superconducting circuits. In a superconductor, electrons do not travel solo. Instead, for complicated quantum-mechanical reasons, they pair up (for the same reasons, the pairs feel no electrical resistance). When they do so, the pairs start behaving like a single particle, superposing proclivities and all. This superparticle can, for instance, in effect be moving in two directions at once. As electrons move, they create a magnetic field. Make a closed loop of superconducting wire, then, and you get a magnetic field which can be facing up and down at the same time. You have yourself a superconducting qubit — or five, the number Martinis has so far managed to entangle.
He has another clever trick up his sleeve. Using a device called a resonator he has been able to transfer information from the circuit to a single photon and trap it in a cavity for a few microseconds. He has, in other words, created a quantum memory. A few microseconds may not sound much, but it is just about enough to perform some basic operations.
The problem with all these approaches is that the quantum states they rely on are fragile, which allows errors to creep in. One way to ensure that they do not scupper the calculation is to encode the same information in several qubits instead of just one. Marcus, Martinis and Laflamme have therefore had to build redundant qubits into their systems. For every “logical” qubit needed to do a calculation, there is a handful of physical ones, all of which need to be entangled.
Michael Freedman is trying to address this problem by taking a different tack. Together with his colleagues at Microsoft’s Station Q research centre, also at UCSB, he is trying to build what he calls a topological quantum computer. This uses a superconductor on top of a layer of an exotic material called indium antimony. When a voltage is applied to this sandwich, the whole lot becomes a quantum system capable of existing in superposed states.
Where Freedman’s qubits differ from Martinis’s is in the way they react to interference. Nudge any electron in a superconducting circuit and the whole lot decoheres. Freedman’s design, however, is invulnerable to such local disruptions thanks to the peculiar way in which energy is distributed throughout indium antimony. The Microsoft team has yet to create a functioning qubit, but hopes to do so soon, and is searching for other materials in which to repeat the same trick.
All of this work is pretty fundamental. Researchers are a long way from creating quantum mainframes, which is how most of them see the future of their fiddly devices, let alone quantum desktops. Martinis thinks that a viable quantum processor is still 10 years away. Yet even this is progress of a sort. When he entered the field two decades ago, he thought that building a quantum processor was “insanely difficult”. Now he says it is merely “very, very hard”. — (c) 2012 The Economist
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