A
fullscale quantum computer could produce reliable results
even if its components performed no better than today's
best firstgeneration prototypes, according to a paper
in the March 3 issue in the journal Nature* by a scientist
at the Commerce Department's National Institute of Standards
and Technology (NIST).
In theory, such a quantum computer
could be used to break commonly used encryption codes,
to improve optimization of complex systems such as
airline schedules, and to simulate other complex quantum
systems.
A key issue for the reliability
of future quantum computerswhich would rely on the
unusual properties of nature's smallest particles
to store and process datais the fragility of quantum
states. Today's computers use millions of transistors
that are switched on or off to reliably represent
values of 1 or 0. Quantum computers would use atoms,
for example, as quantum bits (qubits), whose magnetic
and other properties would be manipulated to represent
1 or 0 or even both at the same time. These states
are so delicate that qubit values would be unusually
susceptible to errors caused by the slightest electronic
"noise."
To get around this problem,
NIST scientist Emanuel Knill suggests using a pyramidstyle
hierarchy of qubits made of smaller and simpler building
blocks than envisioned previously, and teleportation
of data at key intervals to continuously doublecheck
the accuracy of qubit values. Teleportation was demonstrated
last year by NIST physicists, who transferred key
properties of one atom to another atom without using
a physical link.
"There has been a tremendous
gap between theory and experiment in quantum computing,"
Knill says. "It is as if we were designing today's
supercomputers in the era of vacuum tube computing,
before the invention of transistors. This work reduces
the gap, showing that building quantum computers may
be easier than we thought. However, it will still
take a lot of work to build a useful quantum computer."
Use of Knill's architecture
could lead to reliable computing even if individual
logic operations made errors as often as 3 percent
of the timeperformance levels already achieved in
NIST laboratories with qubits based on ions (charged
atoms). The proposed architecture could tolerate several
hundred times more errors than scientists had generally
thought acceptable.
Knill's findings are based
on several months of calculations and simulations
on large, conventional computer workstations. The
new architecture, which has yet to be validated by
mathematical proofs or tested in the laboratory, relies
on a series of simple procedures for repeatedly checking
the accuracy of blocks of qubits. This process creates
a hierarchy of qubits at various levels of validation.
For instance, to achieve relatively
low error probabilities in moderately long computations,
36 qubits would be processed in three levels to arrive
at one corrected pair. Only the toptier, or most
accurate, qubits are actually used for computations.
The more levels there are, the more reliable the computation
will be.
Knill's methods for detecting
and correcting errors rely heavily on teleportation.
Teleportation enables scientists to measure how errors
have affected a qubit's value while transferring the
stored information to other qubits not yet perturbed
by errors. The original qubit's quantum properties
would be teleported to another qubit as the original
qubit is measured.
The new architecture allows
tradeoffs between error rates and computing resource
demands. To tolerate 3 percent error rates in components,
massive amounts of computing hardware and processing
time would be needed, partly because of the "overhead"
involved in correcting errors. Fewer resources would
be needed if component error rates can be reduced
further, Knill's calculations show.
The research was funded in part by the Defense Advanced
Research Projects Agency.
As a nonregulatory agency,
NIST develops and promotes measurement, standards
and technology to enhance productivity, facilitate
trade and improve the quality of life.
*Knill, E. 2005. "Quantum
computing with realistically noisy devices."
Nature. March 3.
