Optics & Photonics News

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 (although intuitively obvious) way to discuss timekeeping. Instead, 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.

Salvatore Siano

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 external factors.