If you come here regularly, you know that quantum computing is a recurrent theme. In fact, if you use the PicoSearch box on the right side to search for "quantum", you'll see that I already wrote 11 columns on the subject.
Today, the Economist talks about building a practical quantum computer.
Building a practical quantum computer will be hard. But another step towards one has just been announced in Nature. Stephan Gulde, of the University of Innsbruck, in Austria, and his colleagues have built a prototype machine whose chief working part is a single atom of calcium, and they have run a program on it.
Conventional computers manipulate binary digits (bits). Quantum computers manipulate “qubits” (quantum bits). A bit represents a numerical value of one or zero. However, owing to the uncertainty principle (which says that you can never have perfect information about the behavior of a very small object) a qubit can represent a one and a zero simultaneously. [..] The problem is manipulating the qubits.
Dr Gulde begins by trapping an electrically charged calcium atom (ion) into what is called a “linear Paul trap”, which uses an oscillating electric field to hold the ion in place. It is then zapped by light from a titaniumsapphire laser. Instead of heating it up, as might be expected, this zapping stimulates the ion to give up energy in the form of further light, thus cooling it.
The computation itself is performed by a further series of laser pulses. These act on both the electronic state and the vibrational state, and the answer is left encoded in the electronic state. It can be read by a further laser pulse, which stimulates the ion to spit it out in the form of light.
The Economist also offered an analogy between the quantum and nonquantum worlds.
In the nonquantum world, checking a coin involves two steps: look at one side of the coin, and then look at the other. Dr Deutsch and Dr Jozsa, however, have worked out a way to use a quantum computer to test an (imaginary) coin in a single step. It involves measuring, after the coin is tossed, a qubit that is an equal mixture of heads and tails. It turns out that an arithmetical sign (plus or minus) in this qubit depends on whether the coin is fair or not  minus is fair, plus is unfair. Measuring the qubit is a onestep process, in effect looking at both sides of the coin at once.
For more details, please read the full article.
Source: The Economist, January 2, 2003
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