National Physical Laboratory

Ytterbium Ion Optical Frequency Standard

Yb Term

Partial term scheme for the
ytterbium-171 ion

The singly charged ytterbium ion is unique among optical frequency standards in that the lowest-lying excited state is the 2F7/2 state, which decays to the 2S1/2 ground state via an electric octupole transition at 467 nm. The 2F7/2 state is extremely long-lived, with an estimated lifetime of around 6 years. Hence the natural linewidth of this transition is of order 1 nHz, and will never be a limit to the performance of the standard. Instead the stability limit is determined by the probe laser linewidth that can be achieved.

The ion is trapped in an endcap trap and laser-cooled on the 2S1/2 (F=1) - 2P1/2 (F=0) transition using light at 369 nm from a frequency-doubled Ti:sapphire laser. Microwave radiation at 12.6 GHz is fed into the trap by an external waveguide to prevent pumping into the F=0 ground state via off-resonant excitation of the 2P1/2 (F=1) level. From the upper state of the cooling transition, the ion can also decay to the metastable 2D3/2 state. Diode laser radiation at 935 nm is therefore used to return the ion to the ground state via the fast-decaying 3D[3/2]1/2 state.

Yb Schematic

Experimental arrangement for the ytterbium ion optical frequency standard

The 467 nm clock transition is driven by a frequency-doubled Ti:sapphire laser, locked via an external acousto-optic modulator (AOM) to a high finesse optical cavity (F=250 000) constructed from ultra-low-expansivity glass. The frequency difference between half the clock transition frequency and the nearest cavity mode is bridged by a double-passed AOM, which is then stepped in frequency to scan the laser over the transition. With the laser tuned to the clock transition frequency quantum jumps are observed in the fluorescence signal at 369 nm. However, a modification of the usual quantum jump technique is required, because once the ion is excited to the 2F7/2 state, it will not decay for a significant length of time. Hence diode laser radiation at 638 nm is used to return the ion to the ground state via the 1D[5/2]5/2 levels.

Yb Spectrum
Spectrum of the 467 nm clock transition
in the ytterbium-171 ion

The high-finesse cavity is housed within a temperature-controlled, evacuated vacuum chamber which is situated on a passive vibration isolation platform, within an acoustically isolating enclosure. A test using a second identical cavity has shown that the linewidth of light locked to the cavity is 5 Hz on a 1 s timescale, broadening to 20 Hz at 100 s. Following frequency doubling, the quantum jump spectrum has a central feature with a linewidth <40 Hz.

With this resolution an absolute frequency measurement has been made, with a fractional uncertainty of around 2 parts in 1014. The uncertainty is limited by statistics and the dominant systematic effect, which is currently the AC Stark shift due to the light at 467 nm. It is expected that, as laser linewidth, and hence also the power required to drive the transition, is reduced, the AC Stark shift will no longer dominate and other systematic shifts will limit accuracy. In particular the uncertainty to which the blackbody shift of 2.5 parts in 1016, can be determined, may set the ultimate reproducibility of the standard.

The ytterbium ion optical frequency standard is also being used in an experiment to search for possible time variation of the fine structure constant.

Last Updated: 25 Sep 2013
Created: 11 Aug 2007

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