91av

TEST tubes of chloroform may sound like unlikely computers but they are the
unwieldy heart of the only quantum computers operating today. But a new idea
that lets engineers place arrays of atoms on a silicon surface could be used to
build the first 鈥渃hip-based鈥� quantum computer, say scientists in the US and
Australia.

Creating ordered arrays of atoms is essential for assembling the tiny
computers of the future. But the current techniques for moving atoms around
don鈥檛 work for some of the most important chemical elements. All that may change
now that scientists have developed a way to create arrays of any type of atom on
any type of base.

In 1990, Don Eigler at IBM鈥檚 Almaden Research Center in California invented a
technique for picking up individual atoms using the tip of a scanning tunnelling
microscope and depositing them in patterns onto a surface. He famously used the
technique to write the letters 鈥淚BM鈥� with xenon atoms on a nickel substrate. The
technique has since been used to create a variety of structures out of many
different atoms.

But Eigler鈥檚 technique does not work well with atoms that bond strongly to
each other, such as phosphorus and silicon, and these turn out to be very
important for computing. 鈥淧hosphorus atoms simply stick to a silicon surface and
you can鈥檛 pick them up again,鈥� says Bruce Kane, a physicist at the University of
Maryland, who has come up with an alternative technique, alongside colleagues at
the University of New South Wales. 鈥淭he goal was to come up with a way of
depositing the atoms without them touching the silicon substrate
产别蹿辞谤别丑补苍诲.鈥�

The idea is to coat the surface of the silicon with a thin film of superfluid
helium and float phosphorus ions on this. Normally the ions would sink to the
bottom of the film and bond with the substrate, but by applying an electric
field to the sample, the ions can be made to float
(see Diagram).

Because of their mutual repulsion, the ions form into a regular
two-dimensional array known as a Wigner crystal. Switching off the electric
field allows the ions to drop onto the silicon, where they bond to the surface
in this pattern. The helium is then evaporated and another layer of silicon is
placed over the array to cement it in place.

Kane now wants to build a quantum computer in which each phosphorus atom in
the array can store and process single bits of quantum information called qubits
(91av, 24 June 2000, p 36).
An array of only 30 phosphorus atoms could act as the heart of a quantum computer
more powerful than today鈥檚 supercomputers.