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IEEE/LEOS Summer Topical Meetings

Optical frequency standards based on ultra-cold strontium atoms

Andrew D. Ludlow, T. Ido, Martin M. Boyd, Tanya Zelevinsky, Sebastian Blatt, M. Notcutt, Seth M. Foreman, Jun Ye

Published Nov 8, 2005• 0 citations
Publisher

Abstract

As a key component of a strontium based optical clock, we have developed an apparatus which allows laser cooling and trapping of more than 10/sup 7/ /sup 88/Sr atoms to temperatures below 1 /spl mu/K. Using this system, we have performed the first high accuracy measurement of the /sup 88/Sr /sup 1/S/sub 0/-/sup 3/P/sub 1/ clock transition using an ultra-cold sample. Detailed descriptions of both the measurement technique and corresponding systematic uncertainties is provided. In addition, by varying the density of the ultra-cold /sup 88/Sr sample over a range of three orders of magnitude we obtain the first definitive measurement of a density-related frequency shift and linewidth broadening of an optical clock transition in an ultra-cold alkaline earth system. Spectroscopy of the /sub 1/S/sub 0/-/sup 3/P/sub 0/ transition is currently underway using this narrow probe laser and an octave spanning femtosecond laser. Our most recent results are presented. The proposed measurement technique, accuracy, and stability of an optical clock based on the EIT (electromagnetically-induced-transparency) measurement in /sup 88/Sr are described.

DOI
10.1109/leosst.2005.1528001

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•Jan 1, 2007•36 citations
As a key component of a strontium based optical clock, we have developed an apparatus which allows laser cooling and trapping of more than 10/sup 7/ /sup 88/Sr atoms to temperatures below 1 /spl mu/K. Using this system, we have performed the first high accuracy measurement of the /sup 88/Sr /sup 1/S/sub 0/-/sup 3/P/sub 1/ clock transition using an ultra-cold sample. Detailed descriptions of both the measurement technique and corresponding systematic uncertainties is provided. In addition, by varying the density of the ultra-cold /sup 88/Sr sample over a range of three orders of magnitude we obtain the first definitive measurement of a density-related frequency shift and linewidth broadening of an optical clock transition in an ultra-cold alkaline earth system. Spectroscopy of the /sub 1/S/sub 0/-/sup 3/P/sub 0/ transition is currently underway using this narrow probe laser and an octave spanning femtosecond laser. Our most recent results are presented. The proposed measurement technique, accuracy, and stability of an optical clock based on the EIT (electromagnetically-induced-transparency) measurement in /sup 88/Sr are described.
By varying the density of an ultracold 88Sr^{88}\mathrm{Sr}88Sr sample from 109{10}^{9}109 to$>{10}^{12}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$, we make the first definitive measurement of the density-related frequency shift and linewidth broadening of the $^{1}S_{0}\mathrm{\text{\ensuremath{-}}}^{3}P_{1}$ optical clock transition in an alkaline earth system. In addition, we report the most accurate measurement to date of the 88Sr^{88}\mathrm{Sr}88Sr $^{1}S_{0}\mathrm{\text{\ensuremath{-}}}^{3}P_{1}$ optical clock transition frequency. Including a detailed analysis of systematic errors, the frequency is $[434\text{ }\text{ }829\text{ }\text{ }121\text{ }\text{ }312\text{ }\text{ }334\ifmmode\pm\else\textpm\fi{}20(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}33(\mathrm{syst})]\text{ }\text{ }\mathrm{Hz}$.
Direct observation and frequency measurement of /sup 1/S/sub 0//spl rarr//sup 3/P/sub 0 /line within an accuracy of 20 kHz is reported. The resonance frequency with an accuracy of /spl sim/ 100 kHz is reported by indirect measurement of the energy difference between /sup 1/S/sub 0/ and /sup 3/P/sub 0/ states. It is performed by successive frequency measurements of atomic transitions with higher linewidth, easier to observe: /sup 1/S/sub 0/ - /sup 3/P/sub 1/, /sup 3/P/sub 1/ - /sup 3/S/sub 1/ and /sup 3/S/sub 1/ - /sup 3/P/sub 0/ transitions. To perform direct observation of the clock transition, atoms are cooled to a temperature of /spl sim/ 2 mK in a magneto-optical trap (MOT). Although the saturation broadening of the transition is orders of magnitude smaller than the Doppler width (1.5 MHz), a detectable signal is achieved by inducing a small leak in the MOT with a laser tuned to resonance. Atoms accumulate in the metastable /sup 3/P/sub 0 /state where they escape the trap. The resonance is detected by a decrease in the MOT fluorescence signal with a contrast of 1%.
Recent work is presented on the development of an optical frequency standard, based on the /sup 2/S/sub 1/2/-/sup 2/D/sub 5/2/ transition at 674 nm in /sup 88/Sr/sup +/. Work on characterising the probe laser is also presented and the observed linewidth of one of the /spl Delta/m=0 Zeeman components of the 674 nm transition is 290 Hz. Two traps have been operated simultaneously and a null offset between them confirmed at the 3 parts in 10/sup 13/ level. The feasibility of cooling and probing /sup 87/Sr/sup +/ is also discussed.
We report on our progress toward the realization of an optical frequency standard referenced to strontium intercombination lines. While first frequency measurement of the weakly allowed 1S 0- 3P 1 transition was done with saturation spectroscopy on a thermal beam, we are preparing the experimental setup for high resolution spectroscopy on ultracold atoms of the doubly forbidden 1S 0- 3P 0 line. Our current setup allows the capture more than 10 7 atoms at 1 μK in 500 ms in two stages of magneto-optical trapping. We also demonstrated the dipole trapping of ultracold strontium isotopes for accurate spectroscopy and collisional measurements. A semiconductor laser source at 698 nm has already been locked to a pre-stabilization cavity obtaining a fast linewidth of less than 1 kHz. Second stage stabilization of the clock laser to ultra-high finesse cavity is in development. Recent measurement of elastic and inelastic cross-sections of the two most abundant 88Sr and 86Sr isotopes are also presented.
Clocks based on optical transitions in ions and atoms are quickly moving to the forefront of the frequency standards field mainly because of the high spec-tral resolution, and therefore the potential stability and accuracy, which can be achieved. In this thesis a new optical clock based on neutral atoms trapped in an optical lattice is presented, demonstrating spectroscopy of the clock transition free of any lineshape or accuracy degradation due to atomic motion. The system simu-lates a single trapped ion clock, but allows use of thousands of atoms for improved signal to noise ratio, and clock stability. High accuracy can also be achieved as the lattice can be designed to shift the energy of the two atomic clock states equally, such that the transition frequency is unchanged. Strontium is a natural candidate for such a clock as it offers an extremely narrow optical transition (∼1 mHz) based on atomic states which are very insensitive to external fields. The strontium level structure allows efficient laser cooling to 1 µK with diode laser sources, and a con-venient wavelength of 813 nm for the zero-differential Stark shift optical lattice. The strontium lattice clock system has allowed observation of high signal-to-noise