ADD salt to your chips and you could end up with a quantum computer鈥攁s long as your chips are made of silicon rather than spuds. The trick sidesteps many of the difficulties involved in manipulating lone atoms to make quantum processors.
Quantum computers are devices that aim to harness the strange quantum properties of atoms or electrons to generate vast processing power. One proposed design involves phosphorus atoms buried in a sliver of silicon with electrodes placed above each atom (New 杏吧原创, 24 June 2000, p 36). The nucleus of each atom has either a clockwise or anticlockwise spin, and this can be used to store a single bit of quantum information. By manipulating the atoms鈥� spins, the electrodes can process this data in a way that allows the chip to carry out calculations.
But getting the phosphorus atoms into place is fiddly. Moving them one by one with atomic force microscopes is time-consuming and difficult, and bombarding the silicon lattice with phosphorus can damage its structure. What鈥檚 more, positioning the electrodes precisely on top of the buried atoms is harder still.
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Now Crispin Barnes and his colleagues at the University of Cambridge think they can do better by using sodium atoms instead of phosphorus. Their trick is based on the idea that sodium atoms are small enough to drift through the silicon lattice. The researchers call them 鈥渉oming atoms鈥� because they move into position under their electrodes automatically.
Getting the sodium ions onto the chip is simple: it鈥檚 dipped in a solution of sodium chloride鈥攃ommon salt. As it dries, some of the sodium ions from the solution penetrate the thin oxide layer that always forms on silicon in air.
The team then deposits tiny electrodes on the surface of the chip and uses them to set up an electric field. This draws the ions deeper into the silicon, and positions just one under each electrode.
鈥淪witch it on and the sodium ions automatically drift into position and bury themselves in the lattice,鈥� says Barnes. Freezing the chip in liquid nitrogen reattaches electrons to the sodium ions, locking them into place. The atoms can then be used for quantum processing.