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Laser Physics

Improved absolute frequency measurement of the 115In+ 5s 2 1 S 0-5s5p 3 P 0 narrowline transition: Progress towards an optical frequency standard

Y. H. Wang, Tao Liu, Rainer Dumke, Alois Stejskal, Yuhan Zhao, Jie Zhang, Z. H. Lu, L. J. Wang, Th. Becker, HERBERT WALTHER

Published Jul 1, 2007• 10 citations
Publisher

Abstract

We report on an improved absolute frequency measurement of the 5s 2 1 S 00-5s5p 3 P 0 narrowline clock transition at 236.5 nm, for a single, trapped, and laser-cooled 115In ion. Using a narrowline laser as the local oscillator, a linewidth of 43 Hz for the transition is resolved. The uncertainty of the transition frequency’s centroid is 18 Hz, leading to a fractional uncertainty of 1.4 × 10−14. For absolute frequency measurement, we use an optical frequency comb locked to a cesium clock as the reference. The transition frequency is found to be 1267402452900967(63) Hz, averaged over 13 days of separate measurements. The accuracy is about 5.0 × 10−14. We discuss possibilities for further improvement.

DOI
10.1134/s1054660x07070171

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arXiv:1401.1008•Applied Physics Express•Dec 18, 2013•50 citations
We report on an improved absolute frequency measurement of the 5s 2 1 S 00-5s5p 3 P 0 narrowline clock transition at 236.5 nm, for a single, trapped, and laser-cooled 115In ion. Using a narrowline laser as the local oscillator, a linewidth of 43 Hz for the transition is resolved. The uncertainty of the transition frequency’s centroid is 18 Hz, leading to a fractional uncertainty of 1.4 × 10−14. For absolute frequency measurement, we use an optical frequency comb locked to a cesium clock as the reference. The transition frequency is found to be 1267402452900967(63) Hz, averaged over 13 days of separate measurements. The accuracy is about 5.0 × 10−14. We discuss possibilities for further improvement.
We report on the current status of the optical frequency standard based on a single 115In+ ion. A single Indium ion is being trapped in a Paul-Strauble type trap and cooled in combination of Doppler cooling and sideband cooling to its lowest vibrational states. To interrogate the1S0-3P0clock transition with high accuracy we demonstrate a Hz-level laser system resonant to this transition. With the current setup we are able to achieve a measured linewidth of the clock transition of 260 Hz. The statistical uncertainty of the absolute frequency measurement is 3×10-13, limited mainly by our reference frequency standard.
The present decade has seen great advances in optical frequency standards following the development of femtosecond laser frequency combs. With this technology, combined with a stable and narrow probe laser source (50-100 Hz linewidth), we made a systematic study of the 5s 2S1/2-4d 2D5/2 (S-D) transition frequency of the 88Sr+ ion as a function of the quantization axis in the trap. This study revealed the presence of systematic shifts caused by residual micromotion in our rf Paul trap and led to an evaluation of those shifts. Also, a method was developed based on the measurement of the Zeeman spectrum of the clock transition to cancel the electric quadrupole shift caused by patch potentials on the trap electrodes. From these results, we also determined the absolute frequency of the S-D clock transition frequency at 674 nm with a fractional frequency uncertainty of 3.4 × 10-14. We have since made improvements to our probe laser system and have observed 5 Hz Fourier transform limited linewidths on a Zeeman component of the S-D transition of the 88Sr+ ion. To observe such a narrow transition it is essential to have a known and well behaved drift of the laser frequency during the few minutes it takes to measure the ion resonance using the quantum jump signal. Our approach to minimize the drift rate and its sensitivity to temperature fluctuations has been to stabilize the cavity at the temperature where its coefficient of thermal expansion is zero. We will present an overview of our recent progress made on the frequency standard based on the laser-cooled and trapped single ion of 88Sr+.
The frequency of the 5s 2S(1/2)-4d 2D(5/2) electric quadrupole clock transition in a single, trapped, laser-cooled 88Sr+ ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cesium standard.
We are investigating the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 0/ transition of a single trapped laser-cooled /sup 115/In/sup +/ ion as a possible optical frequency standard. This line with a natural linewidth of only 1.1 Hz, corresponding to a Q factor of 1.2/spl middot/10/sup 15/, is highly immune to systematic frequency shifts due to external perturbations. For sideband laser cooling and fluorescence detection of the indium ion the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 1/ transition at 230.6 nm is excited. Temperatures below 100 /spl mu/K and a mean vibrational quantum number<1 of the ion in the trap have been reached. For the clock transition a resolution of 3/spl middot/10/sup -13/ (linewidth 350 Hz) has been obtained so far, limited by the laser linewidth. The absolute frequency of the /sup 1/S/sub 0//spl rarr//sup 3/P/sub 0/ transition was measured by making a link to known reference frequencies of molecular iodine and methane using a frequency chain.
We performed spectroscopic observations of the 698-nm clock transition in 87 Sr confined in an optical lattice using a laser linewidth transfer technique. A narrow-linewidth laser interrogating the clock transition was prepared by transferring the linewidth of a master laser (1064 nm) to that of a slave laser (698 nm) with a high-speed controllable fiber-based frequency comb. The Fourier-limited spectrum was then observed for an 80-ms interrogating pulse. We determined that the absolute frequency of the 5s 2 1 S 0 –5s5p 3 P 0 clock transition in 87 Sr is 429 228 004 229 872.0 (1.6) Hz referenced to the SI second.