Boulder,
Colo.—A crucial step in a procedure that could enable
future quantum computers to break today's most commonly
used encryption codes has been demonstrated by physicists
at the U.S. Commerce Department's National Institute
of Standards and Technology (NIST).
As reported in the May 13 issue of the journal Science ,* the NIST team
showed that it is possible to identify repeating patterns in quantum information
stored in ions (charged atoms). The NIST work used three ions as quantum bits
(qubits) to represent 1s or 0s—or, under the unusual rules of quantum physics,
both 1 and 0 at the same time. Scientists believe that much larger arrays of
such ions could process data in a powerful quantum computer. Previous demonstrations
of similar processes were performed with qubits made of molecules in a liquid,
a system that cannot be expanded to large numbers of qubits.
“Our demonstration is important, because it helps pave the way toward building
a largescale quantum computer,” says John Chiaverini, lead author of the paper. “Our
approach also requires fewer steps and is more efficient than those demonstrated
previously.”
The NIST team used electromagnetically trapped beryllium ions as qubits to demonstrate
a quantum version of the “Fourier transform” process, a widely used method for
finding repeating patterns in data. The quantum version is the crucial final
step in Shor's algorithm, a series of steps for finding the “prime factors” of
large numbers—the prime numbers that when multiplied together produce a given
number.
Developed by Peter Shor of Bell Labs in 1994, the factoring algorithm sparked
burgeoning interest in quantum computing. Modern cryptography techniques, which
rely on the fact that even the fastest supercomputers require very long times
to factor large numbers, are used to encode everything from military communications
to bank transactions. But a quantum computer using Shor's algorithm could factor
a number several hundred digits long in a reasonably short time. This algorithm
made code breaking the most important application for quantum computing.
Quantum computing, which harnesses the unusual behavior of quantum systems, offers
the possibility of parallel processing on a grand scale. Unlike switches that
are either fully on or fully off in today's computer chips, quantum bits can
be on, off, or on and off at the same time. The availability of such “superpositions,” in
addition to other strange quantum properties, means that a quantum computer could
solve certain problems in an exponentially shorter time than a conventional computer
with the same number of bits. Researchers often point out that, for specific
classes of problems, a quantum computer with 300 qubits has potentially more
processing power than a classical computer containing as many bits as there are
particles in the universe.
Harnessing all this potential for practical use is extremely difficult. One problem
is that measuring a qubit causes its delicate quantum state to collapse, producing
an output of an ordinary 1 or 0, without a record of what happened during the
computation. Nevertheless, Shor's algorithm uses these properties to perform
a useful task. It enables scientists to analyze the final quantum state after
the computation to find repeating patterns in the original input, and to use
this information to determine the prime factors of a number.
The work described in the Science paper demonstrated the patternfinding
step of Shor's algorithm. This demonstration involves fewer and simpler operations
than those previously implemented, a significant benefit in designing practical
quantum computers.
In the experiments, NIST researchers performed the same series of operations
on a set of three beryllium qubits thousands of times. Each set of operations
lasted less than 4 milliseconds, and consisted of using ultraviolet laser pulses
to manipulate individual ions in sequence, based on measurements of the other
ions. Each run produced an output consisting of measurements of each of the three
ions. The NIST team has the capability to measure ions' quantum states precisely
and use the results to manipulate other ions in a controlled way, before the
delicate quantum information is lost.
The same NIST team has previously demonstrated all the basic components for a
quantum computer using ions as qubits, arguably a leading candidate for a largescale
quantum processor. About a dozen different types of quantum systems are under
investigation around the world for use in quantum processing, including the approach
of using ions as qubits.
The new work was supported in part by the Advanced Research and Development Activity/National
Security Agency.
As a nonregulatory agency, NIST develops and promotes measurement, standards
and technology to enhance productivity, facilitate trade and improve the quality
of life.
*J. Chiaverini, J. Britton, D. Leibfried, E. Knill, M.D. Barrett, R.B. Blakestad,
W.M. Itano, J.D. Jost, C. Langer, R. Ozeri, T. Schaetz and D.J. Wineland. 2005.
Implementation of the semiclassical quantum Fourier transform in a scalable system. Science .
May 13, 2005.
A backgrounder with further information is available at http://www.nist.gov/public_affairs/releases/fourier.htm#back
