The Nobel prize for physics
Trappings of success
Oct 9th 2012, 13:56 by J.P.
IF PHYSICS has a mascot, it must be Schrödinger's cat. Famously alive and dead at the same time, it was born in 1935 of a thought experiment to illustrate the bizarre nature of the quantum world, where particles can persist in two states at once. It proved hard to hunt down in practice, however, because such "superposition" is a fragile phenomenon, easily lost upon any hint of disruption. This year the Nobel committee recognised Serge Haroche, from College de France, and David Wineland, of America's National Institute of Standards and Technology, for their efforts to nab and tame the hypothetical feline—or at least microscopic versions of it.
Dr Haroche and Dr Wineland led two independent teams which, beginning in the 1980s, devised experimental methods to measure and manipulate individual particles while preserving some of their quantum weirdness. Dr Haroche's approach depended on trapping microwave photons by getting them to bounce back and forth between two tiny superconducting mirrors. Crucially, he was able to hold on to them for a tenth of a second, aeons in subatomic terms and long enough to probe their quantum properties.
This he did by introducing a so-called Rydberg atom, tweaked so it curled into a doughnut shape roughly 1,000 times bigger than an ordinary atom, to the mirror-walled cavity. There, it interacted with the photon bouncing around inside. As a result of this interaction, the photon and the atom become entangled. This other strange quantum property means that when the atom is measured, the measurement automatically reveals the state of the photon, while leaving the photon itself intact. By sending a series of Rydberg atoms through the cavity one by one and reading them as they exited, Dr Haroche's team was therefore able to track precisely how a photon behaves when in a superposition without nudging it out of that tenuous state.
Where Dr Haroche used atoms to probe individual photons, Dr Wineland did the opposite. He employed an electric field to trap beryllium atoms, stripped of their electrons, in a vacuum at extremely low temperatures and then pulsed them with laser light. The carefully calibrated pulses served to cool the ions even further, to their lowest possible energy, and to nudge them into a superposition of two different energy states. Such cooling is necessary to remove any residual heat, which causes particles to shed their magical properties. As with Dr Haroche's bouncing photons, the cooled ions remained in superposition long enough to examine them in detail.
Dr Wineland also used them to build the world's most accurate clock. Unlike caesium clocks, whose atomic metronomes oscillate in the microwave range and which have become the standard for precision timekeeping necessary for such things as satellite navigation, Dr Wineland's optical clock ticks at the higher frequency of visible light. One version of it uses two trapped ions that are entangled, so that one ticks unperturbed while the other is used to read the time. The upshot is a device accurate to one part in 1017, a hundredfold improvement on the caesium sort and one that, had it started ticking at the time of the Big Bang 14 billion years ago, would today be off by five seconds. Such precision made it possible to pin down subtle effects of small changes in speed and gravity on the passage of time, as predicted by Albert Einstein's theory of relativity.
Looking ahead, physicists hope that the methods to conjure up, maintain and control superposition and entanglement pioneered by Dr Haroche and Dr Wineland will usher in the era of quantum computers. Such machines would, it is thought, be capable of solving some of the problems which stump today's machines, like finding prime factors of numbers with hundreds of digits or trawling through large databases at astonishing speeds.
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 electrical voltages. Dr Haroche's and Dr Wineland's work makes it possible to use other properties of particles, like ions' energy levels, to construct a quantum analogue of the traditional bit—the qubit. Entanglement, meanwhile, allows more qubits to be added. Each extra qubit in a quantum machine doubles the number of simultaneous operations it can perform. Two entangled qubits permit four operations; three permit eight; and so on. In theory, a 300-qubit computer could perform more simulataneous operations than there are atoms in the visible universe.
In 1995 Dr Wineland's team was the first to demonstrate a quantum computation with two qubits, and others have improved on that result since. The Nobel committee was quick to point out that quantum laptops are not in the offing just yet. But the laureates' efforts have brought them closer to reality.
Thank you,too.Good night.
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