Accurate time standards are important for research and for technology as varied as surveying and telecommunications. Scientists at the National Institute of Standards and Technology in the United States and at the Russian Academy of Sciences' Institute of Laser Physics and Novosibirsk State University in Novosibirsk, Russia have significantly improved the design of optical atomic clocks. The new clock is based on an optical lattice created by an intense near-visible laser beam that creates a line of about 1,000 pancake-shaped wells of laser light. Each of these wells contains about 10 atoms of the heavy metal ytterbium. The lattice is loaded by first slowing down the ytterbium atoms with violet laser light and then further cooling them with green laser light so they can be captured.
Scientists can detect the atoms' "ticks" at 518 THz by bathing them in yellow light at slightly different frequencies until they find the exact frequency, or color, that the atoms absorb best. Optical lattice clocks are not new, but earlier designs used atoms with odd-number atomic masses, which have a nuclear magnetic field, increasing the complexity of the clock. In developing the new clock, researchers found that a small external magnetic field could be combined with the yellow laser light to induce an otherwise "forbidden" oscillation between to energy states in the atoms. This oscillation has a signal-to-noise ratio of 10 and an extremely precise resonance frequency.
In the paper Direct excitation of the forbidden clock transition in neutral Yb atoms confined to an optical lattice, the researchers said this signal to noise ratio should enable a measurement precision of 2 Hz in one second at a clock frequency of 518 THz, supporting a fractional frequency instability of 4 times 10 to the minus 15th power, with one second averaging. This compares with the best existing frequency standards.
The referenced paper has a detailed description of the clock, including illustrations. Also see Magnetic field-induced spectroscopy of forbidden optical transitions with applications to lattice-based optical atomic clocks. Both papers appeared in the March 3 issue of "Physical Review Letters."
For a high-level summary of the research, see the NIST Tech Beat article Experimental Atomic Clock Uses Ytterbium 'Pancakes'.
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