atoms advance quantum chips
Technology Research News
A team of researchers at the University
of New South Wales in Australia has laid the foundation for quantum computer
chips that closely resemble today's mass-produced semiconductor chips.
This is in sharp contrast to today's rudimentary prototype quantum computers,
which are built out of complicated laboratory equipment.
The researchers have placed individual phosphorus atoms at regular intervals
on a silicon surface. The work is the first step in implementing a silicon-based
architecture that uses phosphorus atoms embedded in silicon as quantum
bits, or qubits.
"Our [research is] a demonstration of the controlled placement of single
molecules on a semiconductor surface," said Jeremy O'Brien, a graduate
student at the University of New South Wales.
This was challenging because individual phosphorus atoms readily bond
to silicon, which makes it impossible to align phosphorus atoms by moving
them around on a silicon surface. The researchers got around the problem
by coating the silicon with a one-atom-thick layer of hydrogen and then
using the probe tip of a scanning tunneling microscope to remove individual
hydrogen atoms at regular intervals.
The researchers put phosphorus atoms into the holes left after removing
the hydrogen atoms by exposing the hydrogen-coated silicon to phosphine
gas. Phosphine gas molecules are composed of phosphorus and hydrogen atoms.
The phosphine bonded to the silicon, one molecule to a hole.
This showed that "it is possible to fabricate an atomically precise linear
array of single, phosphorus-bearing molecules on a silicon surface with
the required dimensions for the fabrication of a silicon-based solid-state
quantum computer," said O'Brien.
The researchers were able to position the phosphorus atoms at four-nanometer
intervals, which is smaller than the 20-nanometer intervals required for
the phosphorus-silicon architecture. A nanometer is about 10 hydrogen
"One of the major drawbacks to the [phosphorus-silicon] scheme was [the
need to] to position phosphorus atoms with atomic precision on a silicon
crystal," said Jonathan P. Dowling, supervisor of the quantum computing
technologies group at NASA's Jet Propulsion Laboratory. "This phosphine
idea is really neat, somewhat miraculous, and appears as if it might really
work," he said.
The next step in the process is to cover the phosphine molecules with
another layer of silicon, said O'Brien. "This will require very high-quality
crystal growth to avoid defects which could disrupt the operation of the
quantum computer. We must ensure that the phosphorus qubits incorporate
into the silicon crystal and remain in the ordered atomic array," he said.
Once the phosphorus atoms are sandwiched in silicon, the next challenge
is linking them together and to the outside world. The phosphorus-silicon
architecture calls for a metal electrical contact positioned above each
atom on the top layer of silicon to control the quantum state of the atom
and read the state to determine whether it represents a 1 or a 0. Another
metal contact positioned between two atoms could control the quantum interactions
between them, according to O'Brien.
Quantum computers would be much faster than ordinary computers at certain
tasks like cracking secret codes and searching large databases. Many researchers
in the field say quantum computers are not likely to be ready for practical
use for at least 20 years.
O'Brien's research colleagues were Steven R. Schofield, Michelle Y. Simmons,
Robert G. Clark, Andrew S. Dzurak, Neil J. Curson and N. S. McAlpine of
the University of New South Wales in Australia, Bruce E. Kane of the University
of Maryland, and Marilyn E. Hawley and Geoffrey W. Brown of Los Alamos
National Laboratory. Their research has been accepted for publication
in the journal Physical Review B. The research was funded by the Australian
Research Counsel, the Australian Government, the National Security Agency
and the Advanced Research and Development Activity.
Timeline: >20 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Towards the fabrication
of phosphorus qubits for a silicon quantum computer," posted on the Los
Alamos National Laboratory archive at http://arXiv.org/abs/cond-mat/0104569
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