Written by AZoNanoAug 6 2009

## Raising prospects for building a practical quantum computer, physicists at
the National Institute of Standards
and Technology (NIST) have demonstrated sustained, reliable information
processing operations on electrically charged atoms (ions). The new work, described
in the August 6 issue of Science Express,* overcomes significant hurdles in
scaling up ion-trapping technology from small demonstrations to larger quantum
processors.

In the new demonstration, NIST researchers repeatedly performed a combined
sequence of five quantum logic operations and ten transport operations while
reliably maintaining the 0s and 1s of the binary data stored in the ions, which
serve as quantum bits (qubits) for a hypothetical quantum computer, and retaining
the ability to subsequently manipulate this information. Previously, scientists
at NIST and elsewhere have been unable to coax any qubit technology into performing
a complete set of quantum logic operations while transporting information, without
disturbances degrading the later processes.

"The significant advance is that we can keep on computing, despite the
fact we're doing a lot of qubit transport," says first author Jonathan
Home, a NIST post-doctoral researcher.

The NIST group performed some of the earliest experiments on quantum information
processing and has previously demonstrated many basic components needed for
computing with trapped ions. The new research combines previous advances with
two crucial solutions to previously chronic vulnerabilities: cooling of ions
after transport so their fragile quantum properties can be used for subsequent
logic operations, and storing data values in special states of ions that are
resistant to unwanted alterations by stray magnetic fields.

As a result, the NIST researchers have now demonstrated on a small scale all
the generally recognized requirements for a large-scale ion-based quantum processor.
Previously they could perform all of the following processes a few at a time,
but now they can perform all of them together and repeatedly: (1) "initialize"
qubits to the desired starting state (0 or 1), (2) store qubit data in ions,
(3) perform logic operations on one or two qubits, (4) transfer information
between different locations in the processor, and (5) read out qubit results
individually (0 or 1).

Through its use of ions, the NIST experiment showcases one promising architecture
for a quantum computer, a potentially powerful machine that theoretically could
solve some problems that are currently intractable, such as breaking today's
most widely used encryption codes. Relying on the unusual rules of the submicroscopic
quantum world, qubits can act as 0s and 1s simultaneously, unlike ordinary digital
bits, which hold only one value at any given time. Quantum computers also derive
their power from the fact that qubits can be "entangled," so their
properties are linked, even at a distance. Ions are one of a number of different
types of quantum systems under investigation around the world for use as qubits
in a quantum computer. There is no general agreement on which system will turn
out to be the best.

The NIST experiments described in Science Express stored the qubits in two
beryllium ions held in a trap with six distinct zones. Electric fields are used
to move the ions from one zone to another in the trap, and ultraviolet laser
pulses of specific frequencies and duration are used to manipulate the ions'
energy states. The scientists demonstrated repeated rounds of a sequence of
logic operations (four single-qubit operations and a two-qubit operation) on
the ions and found that operational error rates did not increase as they progressed
through the series, despite transporting qubits across macroscopic distances
(960 micrometers, or almost a millimeter) while carrying out the operations.

The NIST researchers applied two key innovations to quantum-information processing.
First, they used two partner magnesium ions as "refrigerants" for
cooling the beryllium ions after transporting them, thereby allowing logic operations
to continue without any additional error due to heating incurred during transport.
The strong electric forces between the ions enabled the laser-cooled magnesium
to cool down the beryllium ions, and thereby remove heat associated with their
motion, without disturbing the stored quantum information. The new experiment
is the first to apply this "sympathetic cooling" in preparation for
successful two-qubit logic operations.

The other significant innovation was the use of three different pairs of energy
states within the beryllium ions to hold information during different processing
steps. This allowed information to be held in ion states that were not altered
by magnetic field fluctuations during ion storage and transport, eliminating
another source of processing errors. Information was transferred to different
energy levels in the beryllium ions for performing logic operations or reading
out their data values.

The NIST experiment began with two qubits held in separate zones of the ion
trap, so they could be manipulated individually to initialize their states,
perform single-qubit logic operations, and read out results. The ions were then
combined in a single trap zone for a two-qubit logic operation, and again separated
and transported to different trap regions for subsequent single-qubit logic
operations. To evaluate the effectiveness of the processes, the scientists performed
the experiment 3,150 times for each of 16 different starting states. The experimental
results for one and two applications of the sequence of operations were then
compared to each other as well as to a theoretical model of perfect results.

The NIST quantum processor worked with an accuracy of 94 percent, averaged
over all iterations of the experiment. In addition, the error rate was the same
for each of two consecutive repeats of the logical sequence, demonstrating that
the operations are insulated from errors that might have been introduced by
ion transport. The error rate of 6 percent is not yet close to 0.01 percent
threshold identified by experts for fault-tolerant quantum computing, Home notes.
Reducing the error rate is a focus of current NIST research. Another issue in
scaling up the technology to build a practical computer will be controlling
ions in large, complex arrays of traps—work also being pursued in the
NIST group.

There are also more mundane challenges: NIST scientists successfully performed
five rounds of the logic and transport sequence (a total of 25 logic operations
plus 4 preparation and analysis steps), but an attempt to continue to a sixth
round crashed the conventional computer used to control the lasers and ions
of the quantum processor. Nonetheless, the new demonstration moves ion-trap
technology significantly forward on the path to a large quantum processor.

The research was supported in part by the Intelligence Advanced Research Projects
Activity.

*J. P. Home, D. Hanneke, J. D. Jost, J. M. Amini, D. Leibfried and D. J. Wineland.
2009. Complete methods set for scalable ion trap quantum information processing.
Science Express. Posted online August 6.