The ion trap where the main action takes place in the NIST aluminum ion clock. The aluminum ion and partner magnesium ion sit in the slit running down the center of the device between the electrodes. Credit: J. Koelemeij/NIST
Physicists at the National Institute of Standards and Technology (NIST) have built an enhanced version of an experimental atomic clock based on a single aluminum atom that is now the world’s most precise clock, more than twice as precise as the previous pacesetter based on a mercury atom.
The new aluminum clock would neither gain nor lose one second in about 3.7 billion years.
The new clock is the second version of NIST’s “quantum logic clock,” so called because it borrows the logical processing used for atoms storing data in experimental quantum computing.
The logic clock is based on a single aluminum ion (electrically charged atom) trapped by electric fields and vibrating at ultraviolet light frequencies, which are 100,000 times higher than microwave frequencies used in NIST-F1 and other similar time standards around the world. Optical clocks thus divide time into smaller units, and could someday lead to time standards more than 100 times as accurate as today’s microwave standards. Higher frequency is one of a variety of factors that enables improved precision and accuracy.
NIST postdoctoral researcher James
Chin-wen Chou with the world’s most precise clock, based on the vibrations of a single aluminum ion (electrically charged atom). The ion is trapped inside the metal cylinder (center right). Credit: J. Burrus/NIST
Aluminum is one contender for a future time standard to be selected by the international community. NIST scientists are working on five different types of experimental optical clocks, each based on different atoms and offering its own advantages. NIST’s construction of a second, independent version of the logic clock proves it can be replicated, making it one of the first optical clocks to achieve that distinction. Any future time standard will need to be reproduced in many laboratories.
We have constructed an optical clock with a fractional frequency inaccuracy of 8.6 × 10^−18, based on quantum logic spectroscopy of an Al+ ion. A simultaneously trapped Mg+ ion serves to sympathetically laser-cool the Al+ ion and detect its quantum state. The frequency of the 1S0$3P0 clock transition is compared to that of a previously constructed Al+ optical clock with a statistical measurement uncertainty of 7.0 × 10^−18. The two clocks exhibit a relative stability of 2.8×10^−15 −1/2, and a fractional frequency difference of −1.8×10^−17, consistent with the accuracy limit of the older clock.
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