Ytterbium Clock Sets New Stability Mark

An international team of researchers has built a clock whose quantum-mechanical ticking is stable to within 1.6 x 10-18 (a little better than two parts in a quintillion). What does 1.6 x 10-18 look like? It looks like 0.23 seconds in the 4.54-billion-year history of the Earth, or 66 millimeters in the 4.37-light-year distance from Earth to Alpha Centauri. And it may soon usher in an ability to map the interior of the earth by tracking the relativistic changes in clocks ticking on the surface, push terrestrial and space navigation to new levels of precision, and test the laws of physics.

Scientists from the U.S. National Institute of Standards and Technology in Boulder, Colorado, and collaborators at the University of Colorado, the Italian National Institute of Metrological Research, and Turin Polytechnic constructed two clocks based on ytterbium atoms (171Yb) trapped in an optical lattice, spin-polarized, and laser-cooled from 800 Kelvin down to 10 microKelvin (ten-millionths of a degree above absolute zero). Lead author, N. Hinkley, group leader, A.D. Ludlow, and their collaborators describe the device in paper published online by Science.

The path to this breakthrough was bristling with technical challenges such as electromagnetic phenomena that tend to skew frequency measurements. Among them are the Dick effect (interference from the interrogation laser), plus the Stark and Zeeman effects (changes in emitted spectra in response to electric and magnetic fields, respectively), and Doppler shift (changes in light frequency due to an atom’s motion, however small).

The NIST ytterbium clock suspends two clouds of atoms—5000 atoms in each—in optical lattices, which are essentially electromagnetic nets of potential wells created by reflecting a laser beam back upon itself. To prevent the field from producing Stark shifts, the researchers tuned the lattice laser to its “magic” wavelength (759 nm), creating a field that balances the Stark shifts for the two clocks.

Once cooled and stabilized, the clock pumps the contents of the clouds to one of two ground-energy spin states. An interrogating laser then queries both spin states to cancel Zeeman and Stark shifts. Fluorescence reveals the number of atoms originating in the first cloud that are in the ground and excited states, respectively. The device then examines the state of the atoms from the other lattice. The device averages the frequency of each group of atoms over time, and compares them to calculate the clock’s inherent instability.

The precision of this average improves over time (inversely proportional, in fact, to the square root of the sampling time, as is the case with many fluorescent devices). The device attained its 1.6 x 10-18 stability only after 25 000 seconds (6.94 hours) of run time. Though that is a new benchmark, the team thinks it can do better: larger atom clouds, improved methods for reducing interference, and a protocol for alternately loading and examining ytterbium lattices, they say, could allow the clock to reach the same level of stability in runs lasting just 100 seconds. If they, or any group, can manage that, they would likely open up a new class of measurement, with clocks that can gauge not only time, but also space and mass more exactly than ever before.

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