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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

High-resolution spectroscopy of the88Sr+single ion optical frequency standard

P. Dubé, Alan A. Madej, J. E. Bernard, Andrew D. Shiner

Published Aug 29, 2007• 8 citations
Publisher

Abstract

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+.

DOI
10.1117/12.734689

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Science•Nov 18, 2004•302 citations
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 electric quadrupole shift is an important source of systematic uncertainty in several single-ion optical frequency standards. In this paper, we review the electric quadrupole shift cancellation method based on the Zeeman spectrum of the clock transition. The method is demonstrated with an actual frequency measurement of the 5s2S1/2-4d2D5/2transition of88Sr+. This cancellation method also removes shifts from the tensor part of the Stark effect. Further analysis of the data provides an evaluation of the micromotion shifts which are then applied to correct the observed ion clock frequency. Recent improvements to our probe laser system and the observation of Fourier-transform limited linewidths of 5 Hz at 445 THz are also reported
We report our recent progress made with the NRC88Sr+single-ion optical frequency standard. The long-term operation of the standard was improved by actively stabilizing the cooling, repump and clearout laser sources, and by using a femtosecond fiber laser frequency comb to link the probe laser frequency to the microwave time standards. With the femtosecond fiber comb, we have demonstrated continuous operation for a period of eight days and continuous measurement of the probe laser source for three days. Micromotion shifts have been the dominant source of uncertainty in our rf Paul trap because laser beam access is only possible along one axis in the current design. With the aim of reducing to a minimum these shifts, we have constructed an endcap trap designed for minimization of micromotion along three orthogonal axes. We report on the successful trapping of single ions with this endcap trap. Since electrode contamination during trap loading can also induce micromotion shifts as a result of evolving patch potentials, we have increased the trap loading efficiency with photo-ionization.
The authors have designed and implemented a new 674 nm (445 THz) laser system for probing the ultranarrow 5s2S1/2-4d2D5/2reference (clock) transition of the trapped and laser-cooled88Sr+ion. The authors describe briefly the double-stage probe laser system; linewidths of (50 plusmn 9) Hz FWHM were observed for the clock transition with this source. Results on the determination of the temperature of zero thermal expansion coefficient of the reference cavity were also presented. Preliminary measurement of the absolute frequency of the88Sr+clock transition frequency gives a frequency of (444779044095472 plusmn 15) Hz
A single laser cooled ion in a radiofrequency trap can serve as the basis for a highly stable optical frequency standard. We present recent results of our work on single indium ions, using the /sup 1/S/sub 0/ to /sup 3/P/sub 0/ transition at 236.5 nm wave-length as the clock transition. This resonance has a linewidth of only 0.82 Hz where systematic shifts should be controllable on the mHz level. A single In/sup +/ ion is stored in a miniature Paul-Straubel trap and laser cooled to a temperature of about 100 /spl mu/K. The clock transition is excited using a frequency quadrupled Nd:YAG laser. A fractional resolution of 1.3/spl middot/10/sup -13/ has been achieved so far (linewidth of 170 Hz at 1267 THz). The absolute frequency of the clock transition clock transition has been measured with an uncertainty of 2/spl middot/10/sup -13/ using a frequency chain and a methane-stabilized HeNe laser as a reference, that was calibrated with a cesium clock.
Single laser-cooled ions in radiofrequency traps can serve as the basis for highly stable optical frequency standards. Here we present recent results on In+, using the 1S0 yields 3P0 line at 236.5 nm wavelength as the clock transition. This resonance has a natural linewidth of only 0.82 Hz and systematic shifts should be controllable at the mHz level. A single indium ion is stored in a miniature Paul- Straubel trap and laser cooled to a temperature of about 100 (mu) K. The clock transition is excited using a frequency quadrupled 946 nm Nd:YAG laser. A fractional resolution of 1.3 (DOT) 10-13 has been achieved so far (linewidth of 170 Hz at 1267 THz). The absolute optical frequency of the clock transition has been measured with an uncertainty being less than 3 (DOT) 10-13 using a frequency chain and a methane-stabilized HeNe laser that was calibrated with a cesium clock.
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.