This story is a supplement to the feature "Quantum Computing with Ions" which was printed in the August 2008 issue of Scientific American.

One method for building a trapped-ion computer is to connect the ions through their common motions. A string of ions is electrically levitated between two arrays of electrodes. Because the positively charged particles repel one another, any oscillatory motions imparted to one ion (say, by a laser) will move the whole string.

Lasers can also flip the ions’ magnetic orientations, which encode the data carried by the string—an up orientation can correspond to 1, and down can represent 0.

Scaling this system up to larger numbers of ions presents difficulties, however. It appears that longer strings—those containing more than about 20 ions—would be nearly impossible to control because their many collective modes of common motion would interfere with one another. So scientists have begun to develop gridlike traps in which ions can be moved from a string in the system’s memory, say, to another string where data are being processed. The quantum entanglement of the ions allows data to be transferred from one zone of the trap to another.