The magnetic resonance imaging devices
that hospitals use to diagnose illnesses provide detailed pictures of
the insides of the human body by measuring the unique responses of the
atoms and molecules in specific types of tissue to particular sequences
of radio waves and magnetic pulses.
The technology also gives scientists a way to control the spins,
or magnetic orientations of atoms; this ability has led to several prototype
quantum computers. Although nuclear magnetic resonance quantum computer
prototypes have been among the most advanced quantum devices built, such
systems are generally limited to about 10 quantum bits, which is well
short of the thousands needed for practical systems.
Researchers from NTT Basic Research Labs in Japan and the Japan
Science and Technology Agency have built a nuclear magnetic resonance
device that has the potential to overcome the limit because it is small
enough to fit on a computer chip. It could also be tapped to allow nuclear
magnetic resonance devices used in chemistry, biology and medicine to
examine smaller samples, according to the researchers.
Quantum computers use properties like spin to represent the 1s
and 0s of digital information. In theory, quantum computers would be able
to solve certain types of very large problems, including those underpinning
today's encryption technologies, many orders of magnitude faster than
today's classical computers.
The researchers' device measures spin by measuring electrical
resistance across a 200-by-200-nanometer area of semiconductor material
rather than using a centimeter-scale coil to pick up radio waves. This
allows it to control and measure a much smaller number of atomic spins
and to control and measure six distinct types of spin.
The researchers' next step is to fabricate a quantum integrated
circuit by connecting several nuclear magnetic resonance devices. Even
without links to each other, the devices could be used as quantum memory,
according to the researchers.
It will be 10 or 20 years before quantum computers that contain
100 to 10,000 qubits are ready for commercial use, according to the researchers.
The work appeared in the April 21, 2005 issue of Nature (Controlled
Multiple Quantum Coherences of Nuclear Spins in a Nanometer-Scale Device).
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