A Fine Time Was Had by Al+
New experiments show that an optical clock based on an aluminum ion is
the most precise in the world. Chin-wen
Chou and colleagues at the U.S. National
Institute of Standards and Technology
(Boulder, Colo., U.S.A.) demonstrated
aluminum-ion-based clocks with precision
an order of magnitude higher than cesium
fountain clocks, on which the international time standard is based (Phys. Rev.
Lett. 104, 070802).
The additional precision could be
helpful for studying whether properties
that have been assumed to be constant
vary on this scale. For example, the
researchers believe that, with this clock,
they should be able to tell if the fine
structure constant changes over time.
At present, the accepted time and frequency standard is based on a microwave
transition of the cesium atoms—this has
been the standard for 50 years. However,
optical transitions, with their much
shorter wavelengths, offer the potential
for much better accuracy. The transition in the aluminum ion clock is in the
NIST postdoc James Chin-wen Chou with the world’s most precise clock, based on the vibrations of a single aluminum ion trapped inside the metal cylinder (center right).
ultraviolet region. However, higher frequency is
only one factor needed
for improved clocks.
At these very high
precisions, talking about
gaining or losing a second
over a number of years
becomes an unwieldy
obvious) way to discuss
researchers talk about just
how steady the clock is,
using the fractional frequency inaccuracy (
fractional uncertainties), with
smaller numbers being
better. The best atomic
clocks have an accuracy
of about 5 3 10–15.
The optical clock
based on aluminum ions has a fractional
frequency inaccuracy of 8. 6 3 10–18,
more than an order of magnitude better
than a cesium clock. In other words, it
U.S. National Institute of Standards and Technology
DID YOU KNOW?
Laser ablation, used to clean part of the angel on the right, worked better than traditional methods, used on the left two angels. The 15th century painting appears on a wall of the Sagrestia Vecchia in Florence, Italy.
Laser ablation, which is used to remove unwanted tattoos without scarring skin, is now helping to preserve great sculptures, paintings and other works
of art. Salvatore Siano and Renzo Salimbeni of the Applied Physics Institute,
CNR (Florence, Italy), describe how laser ablation can clean the surface of some
artworks with less damage than mechanical and chemical techniques (Acc.
Chem. Res., doi: 10.1021/ar900190f). The method can also be used underwater
to clean archeological artifacts that would degrade if exposed to air, such as
coins from shipwrecks.
would neither gain nor lose one second
in about 3. 7 billion years.
Both aluminum and magnesium ions
are confined in an electromagnetic trap.
The researchers couldn’t cool the Al+ ion
directly, so the researchers laser-cooled
the Mg+ until both types of ions have
been cooled to close to absolute zero.
They looked at the resonant frequency
at which Al+ jumps to a higher-energy
state (from 1S0 to 3P0). The smaller the
variation in repeated measurements, the
more precise the clock.
The energy-level transition in the
Al+ ion is well-suited to the applications
because it doesn’t change much when its
electromagnetic environment changes,
or when the temperature changes. But
although it is stable, its properties cannot
be detected easily with lasers. Therefore,
the Mg+ ion also used to signal the Al+
ion’s quantum state.
The next step is to see if the clocks
can be widely reproduced and show
themselves to be stable over the long
term and resistant to changes due to
Yvonne Carts-Powell ( email@example.com) is a
freelance science writer who specializes in optics