Team makes first quantum computer chip

by MIKE MARTIN, UPI Science Correspondent

NEW SOUTH WALES, Australia, May 7 (UPI) - A team of scientists from the United States and Australia has accomplished a remarkable feat - controlled placement of single phosphorous atoms on a silicon surface to fabricate the world's first quantum computer chips.

The ten-member team of physicists and materials scientists from the University of New South Wales, Los Alamos National Laboratory, and University of Maryland first coated an ultra-clean, ultra-pure silicon surface with a thin layer of hydrogen. Using the sub-nanometer-size tip of a scanning tunneling microscope, the team carefully removed a precisely-spaced array of individual hydrogen atoms, forming small holes in the surface of the hydrogen coating. They next sent a stream of phosphine gas over the silicon. Individual phosphorous molecules from the gas settled into the hydrogen holes, bonding with the exposed silicon. Finally, the team drove off the hydrogen layer by heating it or bombarding the hydrogen atoms with free electrons, leaving a planned array of phosphorous atoms dotting the silicon surface.

"The nuclear or electron spins of the single phosphorus atoms form qubits," the basic building blocks of a solid-state quantum computer, explained Jeremy O'Brien, one of the team's researchers from the Center for Quantum Computer Technology at the University of New South Wales. "We have shown it is possible to fabricate an atomically precise linear array of phosphorus atoms on a silicon surface for the fabrication of a silicon-based, solid state quantum computer."

Other approaches to "qubit" construction include ion trap and nuclear magnetic resonance systems, O'Brien explained. Neither method, however, offers the scalability or ease of mass-production inherent in the phosphorous-silicon method.

Team researcher Marilyn Hawley told United Press International the spin interactions between the phosphorous atoms communicate information directly to the silicon, and phosphorous is a normal "dopant," or commonly-used silicon additive. Nonetheless,
"nobody else has tried this approach before," Hawley explained from her office at the Los Alamos National Laboratory in New Mexico. Researchers at the University of Illinois have used a scanning tunneling microscope to move molecules before, however.
"We charge the tip, which has atomic-scale resolution, to induce vibrations on the hydrogen atoms that break their bonds with the silicon," Hawley explained. The end result - minute molecular tweezers that literally "pluck" the individual hydrogen atoms from the silicon surface, creating tiny holes for the deposition of phosphorous.

The use of this novel technique to marry phosphorous and silicon is "more than sound said Arthur Rheingold, director of the University of Delaware's x-ray crystallographic facility and an inorganic chemist who focuses on solid-state materials. "From a solid-state materials point of view, the successful end result of this work does not surprise me," Rheingold said from Newark.

The technique's inventor, University of Maryland physicist Bruce Kane, says one last challenge remains.

"We have to figure out a way to actually encapsulate the phosphorous qubits within the silicon's crystalline lattice, without moving the phosphorous atoms from their carefully constructed array" Kane told UPI from College Park. "Right now they are just sitting on the surface."

The team's results are due to appear in the journal Physical Review B by early summer.