BOULDER, Colo.—Physicists at the Commerce Department’s
National Institute of Standards and Technology (NIST) have
used the natural oscillations of two different types of charged
atoms, or ions, confined together in a single trap, to produce
the “ticks” that may power a future atomic clock.
As reported in the July 29 issue of Science,*
the unusual tandem technique involves use of a single beryllium
ion to accurately sense the higher-frequency vibrations
of
a single aluminum ion. The NIST group used ultraviolet lasers
to transfer energy from the aluminum’s vibrations to
a shared “rocking” motion of the pair of ions,
and then detected the magnitude of the vibrations through
the beryllium ion. The new technique solves a long-standing
problem of how to monitor the properties of an aluminum ion,
which cannot be manipulated easily using standard laser techniques.
The tandem approach
might be used to make an atomic clock based on optical
frequencies, which has the potential to be
more accurate than today’s microwave-based atomic clocks.
It may also allow simplified designs for quantum computers,
a potentially very powerful technology using the quantum
properties
of matter and light to represent 1s and 0s.
“Our experiments show that we can transfer information
back and forth efficiently between different kinds of atoms.
Now we are applying this technique to develop accurate optical
clocks based on single ions,” said Till Rosenband of
NIST’s laboratories in Boulder, Colo.
Today’s
international time and frequency standards measure naturally
occurring oscillations of cesium atoms that
fall within the frequency range of microwaves, about 9 billion
cycles per second. By contrast, optical frequencies are
about
100,000 times higher, or about one quadrillion cycles per
second, thus dividing time into smaller units. Aluminum
may
offer advantages over other atoms, such as mercury, being
considered for optical atomic clocks.
Building a clock based on aluminum ions has been impractical
until now because this atom fails to meet three of four requirements.
It does oscillate between two different energy states at a
stable, optical frequency that can be used as a clock reference.
However, aluminum cannot be cooled with existing lasers, and
its quantum state is difficult to prepare and detect directly.
The Science paper describes how beryllium—a
staple of NIST research on time and frequency standards as
well as quantum computing—can fulfill these three requirements
while the aluminum acts as a clock.
In the NIST experiments,
the two ions were confined close together in an electromagnetic
trap. The beryllium ion was
laser cooled and slowed to almost absolute zero temperature,
which helped to cool the adjacent aluminum ion. Then the
scientists
used a different laser to place the aluminum ion in a special
quantum state called a “superposition,” in which,
due to the unusual rules of quantum physics, the ion is in
both of its clock-related energy levels at once. More laser
pulses were used to convert this clock state into a rocking
motion, which—because of the physical proximity of the
two ions and the interaction of their electrical charges—was
shared by the beryllium ion. As the two ions rocked together
in a coordinated fashion, scientists applied two additional
laser beams to convert this motion into a change in energy
level of the beryllium ion, which was then detected.
When the information
is transferred between the two ions, they are briefly “entangled,” another unusual
phenomenon of quantum physics in which the properties of physically
distinct particles are correlated. A logic operation borrowed
from quantum computing was used to transfer the aluminum’s
quantum state to the beryllium. Logic operations are similar
to “if/then” statements in which the outcome depends
on the initial state. For instance, if the aluminum’s
original state was at the lowest energy level, then no information
was transferred. But if the original state was at a higher
level, then energy was transferred to the beryllium in a
proportional
amount.
By repeating the
experiment many times, with different laser frequencies
creating a variety of superposition states in
the aluminum, scientists could determine its “resonant”
or characteristic frequency extremely accurately. This is
the frequency of an internal vibration of the aluminum atom,
which can be used as the “ticks” of an atomic
clock.
The tandem technique could be used to investigate the potential
of various atoms, such as boron and helium, for use in optical
atomic clocks, according to the paper. The technique also
could be used in quantum computing experiments to distribute
information between different types of ions or atoms. Because
different atoms respond to different frequencies of light,
this could improve control of ions or atoms within a potential
future quantum computer. Information about NIST research in
this field is available at http://qubit.nist.gov.
The work described in Science was supported in part
by the Office of Naval Research and the Advanced Research
and Development Activity/National Security Agency.
As a non-regulatory agency, NIST develops and promotes measurement,
standards and technology to enhance productivity, facilitate
trade and improve the quality of life.
*P.O. Schmidt, T. Rosenband, C. Langer, W.M. Itano, J.C.
Bergquist , D.J. Wineland. Spectroscopy using quantum logic.
Science. July 29, 2005.
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