Science | DOI: 10.1145/1364782.1364787
Researchers are optimistic, but a practical device is years away.
SINCE qUANTUM AlGORITHMS
and architectures will ultimately need hardware
on which to run, we’ve explored how the principal
experimental efforts are striving to
produce it. Even 15 years ago, a quantum computer was generally viewed
by computer scientists and physicists
alike as an intriguing but probably unattainable theoretical curiosity. But interest exploded in 1994 after Peter Shor,
then at Bell Laboratories (now at MIT),
published his famous quantum factoring algorithm capable of undermining
widely used cryptosystems that relied
on the difficulty of factoring large numbers. Today, several thousand physics,
computer science, and engineering
researchers in more than 100 groups
in universities, institutes, and companies around the world are exploring the
frontiers of quantum information, encompassing quantum computing, as
well as recently commercialized quantum cryptography and quantum tele-portation communication techniques.
Accelerating progress on virtually all
fronts in this worldwide research community is yielding confidence that a
practical quantum computer is indeed
Quantum computing’s potential
has always been tantalizing: Exponentially scalable computing power that
could solve problems beyond the capabilities of conventional computers.
The key is exploiting the superposition of quantum-entangled information units, or qubits. But the research
challenges are daunting: How to create
and reliably compute with the qubits,
which require the seemingly mutually
exclusive conditions of exquisite classical control while being isolated from
any external influences that could destroy the entanglement.
The computing power of a quantum
computer grows exponentially with the
number of qubits it uses. Dozens to
hundreds of qubits will be needed for a
quantum computer to solve interesting
problems using quantum algorithms
(along with appropriate quantum er-ror-correction techniques needed to be
sure the answer is correct). The qubits
must also be connected by quantum
communication channels into logic
gates that can be manipulated to implement the algorithms.
However, merely having and connecting qubits is not sufficient for a
quantum computer. They must remain
entangled long enough to complete the
number of gate operations required by
ing. Typically, the qubit is a two-level
motion mode for a trapped ion. The
modes are modulated by laser pulses.
The ion motion acts like a data bus,
and gates are implemented by modulating neighboring ions.
“Our decoherence times can be up
to 10 minutes—very long compared
with other quantum computing techniques,” says Dave Wineland of the
National Institute of Standards and
Technology, Boulder, CO. “But our
gates are rather slow, about five microseconds for our two-qubit gates.” Since
Miniature ion trap manufactured by Sandia National Laboratories.
the algorithm and mandatory error correction. Faster gate operation, higher
fidelity (percentage of gate operations
completed correctly), and greater er-ror-correcting efficiency can speed the
calculation or reduce the number of
qubits needed to solve the problem.
More than a dozen different ways
of creating qubits—each with its own
strengths and challenges—have been
developed to date. The following is a
rundown of the leading candidates:
Ion traps use electrical and/or magnetic fields and laser-cooling to create
a “pseudo-molecule” quantum register with micron-scale inter-ion spac-
factoring a 100-to-200-digit number
would require a million operations,
even error-free implementation would
take far longer than the qubit could be
maintained. Researchers, led by Rainer
Blatt of the Institut fur Experimental-physik Universtität Innsbruck in Austria, recently set the record for qubit
Integrating CMOS chips with ion
traps is a recent innovation that permits
quantum communication but uses
classical control and measurement.
One design created at Lucent by Richard Slusher (now at the Georgia Tech
Quantum Institute) and Jungsang Kim