91av

A SIMPLE row of phosphorus atoms embedded in a sliver of ordinary silicon
could help create a hugely powerful quantum computer. 鈥淢any people thought that
the placement of single phosphorus atoms could be a show-stopper,鈥� says Jeremy
O鈥橞rien from the Centre for Quantum Computer Technology at the University of New
South Wales in Sydney. 鈥淏ut we have demonstrated that it is possible.鈥�

Conventional digital computers shuffle around bits of information that are
either in an 鈥渙n鈥� or an 鈥渙ff鈥� state. Quantum computers will exploit the fact
that a quantum bit, or 鈥渜ubit鈥�, can exist in an infinity of states between on
and off to achieve undreamed-of computing speeds.

Individual atoms are the obvious candidates for qubits. The problem is to get
them into a state where they can be both controlled and protected from outside
interference. Physicists have coaxed a few atoms into exotic states that fit the
bill, but they didn鈥檛 make much of a computer.

In 1998, Bruce Kane of the University of Maryland proposed a new scheme:
embedding phosphorus atoms inside a silicon crystal (91av, 24
June 2000, p 36). His architecture could incorporate many qubits, with the added
advantage that it would be easy to build such a device into conventional
microchip circuits.

Now O鈥橞rien and his team have got partway to creating it, with a row of
phosphorus atoms, spaced just nanometres apart, in a pure silicon crystal
surface, as they explain in a forthcoming issue of Physical Review B.
The team started with a clean, atomically flat silicon surface in an ultra-high
vacuum to stop the silicon atoms combining with oxygen. Then they covered the
surface with a layer of hydrogen atoms.

Using the superfine tungsten tip of a scanning tunnelling microscope, they
plucked out single hydrogen atoms where they wanted the qubits to be. To get
phosphorus into the holes, the researchers exposed the surface to phosphine gas
(PH₃). One phosphine molecule bonded with each exposed silicon atom.
鈥淭he idea is that the phosphorus atoms replace silicon atoms in a silicon
crystal,鈥� says O鈥橞rien.

The team must now work out a way of growing more silicon over the phosphorus
atoms, to enclose them within a crystal. 鈥淭he prospects for achieving this next
step look very promising,鈥� O鈥橞rien says.

But 鈥渢he hurdle is not completely passed鈥�, warns David DiVincenzo, a quantum
computing expert from IBM鈥檚 T. J. Watson Research Center in Yorktown Heights,
New York. 鈥淎nd it is only one of many hurdles on the way to the quantum
肠辞尘辫耻迟别谤.鈥�