(now at Duke) is an ion-trap analogue
to the electron-based charge-coupled
device chips used in digital cameras.
“This design gives addressability to an
enormous number of ions,” says MIT’s
Isaac Chuang.
Quantum dots are a popular solid-state host for qubits. Lieven Vandersypen’s research group in Delft, The
Netherlands, reported last November
that it had used an alternating electrical field to control single electrons
contained in gallium arsenide quantum dots. Electrical control is more
selective than the previously used magnetic fields. Their fidelity of flipping a
single-electron spin (a simple gate) is a
little lower than the 73% attained with
magnetic-field control but is expected
to increase as they gain experience. Future research will venture into no-spin
hosts, such as silicon and carbon (
car-bon- 12, nanotubes, graphene), that are
expected to have much longer decoherence times.
Linear optic qubits are created by simultaneously producing forward and
backward photons and encoding their
logical states into vertical and horizontal polarizations. This approach has the
advantage of long decoherence times
and compatibility with fiber optics but
needs higher photon-creation and -de-tection efficiencies. Last December,
Andrew White’s group from the University of Queensland reported that it
had used a linear optic circuit involving
four qubits to find the prime factors of
15 ( 5 and 3) thus demonstrating that
system’s ability to perform the core
processes required for implementing
Shor’s algorithm.
In April, Prem Kumar of Northwestern University announced a quantum
gate created within an optical fiber.
A few years ago, Kumar showed that
photons can remain entangled within
a fiber for a distance of 100 kilometers.
The recent result will be useful in creating quantum repeaters for a distributed quantum information network.
Superconducting qubits can be made
in three types: charge, flux, and phase.
Each uses excitation states of Josephson junctions: two superconductor
pieces separated by an insulator thin
enough for Cooper pairs of electrons
to tunnel across. This approach is scalable, since superconductivity enables
fast control and readout and large,
more than a dozen
different ways
of creating qubits
have been developed
to date.
controllable couplings between widely
separated qubits. However, it also requires extremely low temperatures—
milliKelvins—and tends to have short
decoherence times (to date only a few
microseconds). John Martinis’s group
at the University of California, Santa
Barbara, recently measured single-qubit fidelity of 98% in a phase-qubit
system. And Robert Schoelkopf’s
group at Yale developed a “transmon”
qubit 300 microns long and very stable
against noise.
Hybrid approaches combine the best
features of their parents. Chuang’s research group is integrating ion traps
into superconducting qubits. “Ion traps
are very hard to connect to anything,”
Chuang says. “It would be really nice to
have an ion trap with wires coming in
and out.” Vandersypen says that as ion
traps get smaller they might eventually
look similar to quantum dots.
D-Wave Systems, a venture-backed
company based in Vancouver, BC,
made news twice last year when it announced the operation of a 16-qubit
(in February) and 28-qubit (in November) “adiabatic” quantum computer.
However, many scientists are skeptical of both the claims and their importance. The adiabatic method, which
is a quantum version of simulated annealing, involves slowly evolving a system toward the solution. Skeptics say
this approach will prove to be neither
fault-tolerant nor scalable. Although
the company revealed scant scientific
detail about its approach, its president
and CEO, Geordie Rose, says it has no
proof yet of entanglement—the hallmark of quantum computation.
Diamond-based systems are an intriguing recent entrant. The qubit is the
spin state of a nitrogen impurity adjacent to a vacancy in the carbon crystal.
Stemming from Tom Kennedy’s 2003
research at the Naval Research Laboratory, “nitrogen-vacancy color centers”
have two compelling advantages: their
spin state can be both initialized and
read out optically at room temperature
and weak spin-orbit coupling in diamond makes this qubit well-decoupled
from its environment. Several groups
have jumped in to study this system,
and its decoherence time has increased
from about 50 microseconds to nearly
a millisecond.
Challenges include devising ways to
control individual qubits and couple
them together. But David Awschalom
of the University of California, Santa
Barbara, is optimistic. “If you had told
me,” he says, “a few years ago that you
were going to try to control a single
electron at gigahertz frequencies in a
solid-state material at room temperature, I’d have said, ‘Good luck!’ Now
we’re doing just that.”
This system is also useful as an accessible test bed for studying spin interaction in solid-state materials that
may contribute to the success of other
systems and quantum physics knowledge in general. Awschalom’s group recently watched quantum information
from a single “nitrogen-vacancy center” spin disappear into the “bath” of
spins associated with the much more
common nitrogen impurities not associated with vacancies…then reappear. “In quantum physics, this is a big
deal,” Awschalom says. “It’s an age-old
problem. There have been 1,000 theory
papers on this, but no experiments.”
The recent across-the-board progress, however, has stimulated optimism
in the eventual success and impact of
quantum computing inconceivable 15
years ago. For example, IBM Research’s
David DiVincenzo, who proved in 1995
that quantum algorithms could be
executed using only two-qubit operations and later devised seven widely accepted criteria for a practical quantum
computer, says, “I’m confident that
the quantum computer will eventually
change the world and will deeply influence how information processing will
be done in the future.”
Michael Ross writes about science and technology from
San Jose, CA.
Mark Oskin, University of Washington, Seattle,
contributed to this article.