demo does tricky computing
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Quantum computers can theoretically solve
problems that are beyond even the most powerful possible classical computer
-- like cracking secret codes -- by using the bizarre properties of quantum
particles to search through large numbers of possible answers at once.
Scientists from IBM Research and Stanford University have built a quantum
out of seven atoms and used the computer to show that factoring the
number 15 results in the numbers 3 and 5.
Though seven atoms doesn't sound like a lot and factoring 15 is not a
big problem, the device is something of a milestone in quantum computing.
Seven atoms constitute a large device by the standards of the prototype
quantum computers built to date, and running a factoring algorithm on
the atoms shows that they can be controlled well enough to process information.
The researchers' device is unlikely to lead directly to a practical quantum
computer, but their results could make it easier to design and build quantum
computers in general. "Showing that we can factor 15 with a quantum computer
is akin to how researchers demonstrated early electronic computers calculating
digits of the number Pi," said Isaac L. Chuang, now an associate professor
at the Massachusetts Institute of Technology. "It is a milestone, but
not a useful feat in and of itself."
The researchers' quantum computer consisted of five fluorine and two carbon
atoms that were part of a molecule suspended in a test tube of liquid.
Particles like atoms and electrons spin either up or down, similar to
a top spinning clockwise or counterclockwise, and these spin directions
can represent the ones and zeros of computing.
The researchers turned these atomic quantum bits on and off with a series
of carefully timed radio wave pulses that reversed the spins of the atoms.
This nuclear magnetic resonance (NMR) quantum computing method is based
on the same technology used in MRI medical imaging machines.
What makes quantum bits, or qubits,
more powerful than regular computer bits is that when quantum particles
are isolated from the environment and cannot be observed, they enter the
quantum mechanical state of superposition, which means they are in some
mixture of both spin up and spin down. This allows a qubit to represent
both one and zero at the same time, and a relatively small number of qubits
to represent many numbers at once.
Particles can also be linked, or entangled. When changes are made to one
entangled particle, they all change the same way regardless of the physical
distance between them, as long as they remain in superposition. Using
this bizarre property, quantum computers can theoretically examine every
possible answer to a problem with one series of operations rather than
having to check each individually, which means they could solve problems
that are beyond the capabilities of the most powerful classical computer
The way the researchers simulated, designed and operated their computer
is probably more significant than what they did with it. "[That] we know
how to accurately model errors occurring to large-scale, complicated quantum
information processing systems will be the most useful technical component
of our achievement," said Chuang.
Researchers generally agree that liquid nuclear magnetic resonance is
unlikely to lead to practical quantum computers because it is probably
not possible to make NMR quantum computers much bigger than seven qubits.
However, the way the researchers use the spin of the atoms to compute
is compatible with many quantum computer designs, including those based
on semiconductor devices. "The methods we demonstrated for controlling
these spins... will generally be how future quantum information processing
machines are controlled and programmed," said Chuang.
The research "is an exquisite demonstration of control over complex pulse
sequences combined with a growing bag of tricks for compiling quantum
computing circuits," said Daniel Lidar, an assistant professor of chemistry
at the University of Toronto. "There is no doubt that these techniques...
will be useful for eventual scalable solid-state quantum computing implementations."
The researchers' experiment is one of only a small number that have implemented
such complex algorithms, said Emanuel Knill, a mathematician at Los Alamos
National Laboratory. "The real significance is in the demonstration of
techniques for the control of quantum computers. Any other comparably
complex algorithm with a definite and verifiable answer can serve this
purpose," he said.
Unfortunately, the researchers did not provide the scales necessary to
compare their data, said Knill. "This makes it impossible to determine
how well their experiment worked and how well the measured [results] compared
to simulation. As a result, the value of this contribution as a demonstration
of quantum control is significantly lessened," he said.
According to many researchers, it is likely to be at least 20 years before
practical quantum computers can be built.
There is also a chance that practical general-purpose quantum computers
will never be built, said Chuang. "Classical computing itself is growing
in performance in leaps and bounds, and in terms of raw computational
power, quantum computers may never be competitive," said Chuang.
Chuang's research colleagues were Lieven M. K. Vandersypen and Mathias
Steffen of Stanford University and IBM Research, and Gregory Breyta, Costantino
S. Yannoni and Mark H. Sherwood of IBM Research. They published the research
in the December 20/27, 2001 issue of the journal Nature. The research
was funded by IBM and the Defense Advanced Research Projects Agency (DARPA).
Timeline: 20 years
Funding: Corporate; Government
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Experimental realization
of Shor's quantum factoring algorithm using nuclear magnetic resonance,"
Nature, December 20/27, 2001
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