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Conference on Precision Electromagnetic Measurements

Experimental confirmation of micromotion shift cancellation via operation at trap magic frequency and reduction in evaluated systematic uncertainties for the 88Sr+ optical frequency standard at 445 THz

Alan A. Madej, P. Dubé, Maria Tibbo, J. E. Bernard

Published Aug 1, 2014• 0 citations
Publisher

Abstract

We describe recent experiments and results which show the effective cancellation of micromotion induced systematic shifts for the 445-THz reference transition in a trapped and laser cooled88Sr+single ion reference. Using two different ion trap systems based on a Paul and endcap design, the critical or magic frequency at which micromotion effects cancel has been effectively measured at 14.4 MHz. This work also allows an improved determination of the polarizability difference of the ground and excited states and an improvement in the evaluated blackbody shifts to the low 10-17level.

DOI
10.1109/cpem.2014.6898259

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 29, 2007•8 citations
We describe recent experiments and results which show the effective cancellation of micromotion induced systematic shifts for the 445-THz reference transition in a trapped and laser cooled88Sr+single ion reference. Using two different ion trap systems based on a Paul and endcap design, the critical or magic frequency at which micromotion effects cancel has been effectively measured at 14.4 MHz. This work also allows an improved determination of the polarizability difference of the ground and excited states and an improvement in the evaluated blackbody shifts to the low 10-17level.
As part of the ongoing world-wide effort in improving time and frequency references, a high accuracy optical frequency standard was developed using the electric quadrupole allowed clock transition at 445 THz (674 nm) in a trapped and laser cooled 88Sr+ion. An ion trap system of the endcap design has been recently evaluated with a fractional frequency uncertainty which surpasses the accuracy of the current realization of the SI second. This thesis seeks to further evaluate the limiting systematic shifts of the device by optimizing a second ion trap reference based on a rf Paul trap design, which was then compared with the endcap trap reference frequency. The comparison of the two ion traps' reference frequencies confirmed an overall offset of -0.36 pm 0.08 Hz at the 445 THz reference frequency corresponding to a fractional frequency offset of 8 x 10^-16.
An overview of the key methods used to reduce the uncertainty of the systematic shifts of the strontium ion optical frequency standard is presented. An uncertainty of 10 mHz at a clock frequency of 445 THz is obtained, limited by the uncertainty of the blackbody radiation shift. The uncertainty estimates of other shifts such as micromotion and electric quadrupole shifts are on the order of 1 mHz or less. The effectiveness of the methods used to reduce or cancel the shifts is illustrated with experiments comparing two ion traps. One has strong micromotion levels while the other, the reference trap, provides good control over the shifts. Despite nominal shifts of 70 Hz in the trap with large micromotion levels, the agreement with the reference trap is at the sub-Hz level.
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 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+.