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European Frequency and Time Forum

Progress towards a strontium single-ion optical clock with mHz uncertainty

P. Dubé, Bin Jian, Alan A. Madej

Published Apr 1, 2016• 0 citations
Publisher

Abstract

The strontium ion clock developed at the National Research Council of Canada has reached an uncertainty level of 5 mHz in recent years. This low level of uncertainty was achieved in great part by using methods that harness specific properties of the clock transition that allow either a very high level of cancellation or a strong suppression of frequency shifts that would otherwise dominate the uncertainty budget. The shifts controlled by these methods are the electric quadrupole shift, the tensor Stark shifts, and the micromotion shifts. With these methods in place, the main limitation to the accuracy of the clock transition frequency is the uncertainty in the evaluation of blackbody radiation (BBR) field. In this paper, we review the methods used to control the shifts to the 5 mHz level and present our plans for the evaluation of the BBR field that is expected to reduce the total uncertainty to the mHz level.

DOI
10.1109/eftf.2016.7477797

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The strontium ion clock developed at the National Research Council of Canada has reached an uncertainty level of 5 mHz in recent years. This low level of uncertainty was achieved in great part by using methods that harness specific properties of the clock transition that allow either a very high level of cancellation or a strong suppression of frequency shifts that would otherwise dominate the uncertainty budget. The shifts controlled by these methods are the electric quadrupole shift, the tensor Stark shifts, and the micromotion shifts. With these methods in place, the main limitation to the accuracy of the clock transition frequency is the uncertainty in the evaluation of blackbody radiation (BBR) field. In this paper, we review the methods used to control the shifts to the 5 mHz level and present our plans for the evaluation of the BBR field that is expected to reduce the total uncertainty to the mHz level.
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.
We present the current status of the Sr+optical atomic ion clock at the National Research Council of Canada including the study of the quantum projection noise (QPN) limited stability and the progress on the evaluation of the blackbody radiation (BBR) shift uncertainty. The latest QPN limited stability of NRC's Sr+ clock is 3 × 10-15at 1 sec averaging time. The fractional evaluated accuracy of the Sr+ ion clock has reached 1.2 × 10-17and is now dominated by the BBR shift uncertainty, the improved evaluation of which is currently in progress.
The accuracy of state-of-the-art optical atomic clocks has surpassed that of microwave clocks by more than two orders of magnitude. This revolution in accuracy has prompted international dialogue concerning the redefinition of the SI second away from the current cesium standard. Optical clocks employing a single trapped ion are promising candidates for realizing this redefinition as they offer exceptional fractional frequency uncertainty. However, there is still room for improvement: the leading contributions to the uncertainty of most single-ion clocks, such as the 88Sr+ clock operated by the National Research Council of Canada (NRC), are the blackbody radiation shifts and collisional frequency shifts. The uncancelled blackbody radiation shift dominates both the total shift and uncertainty of the existing room temperature strontium ion clocks. To address these sources of inaccuracy, I contributed to developing a strontium ion trap for operation at 4 K. Operating at cryogenic temperatures suppresses the blackbody radiation shift by eight orders of magnitude and collisional shifts by at least two orders of magnitude, enabling substantial improvements to the performance of single-ion clocks. The proposed cryogenic clock is capable of achieving fractional frequency uncertainty below 10−19. This thesis describes the design, construction, and evaluation of the cryogenic ion trap towards realizing a more accurate atomic clock.
Progress of the 40Ca+ ion optical clock based on the 42S1/2 – 3d 2D5/2 electric quadrupole transition is reported. By setting the drive frequency to the “magic” frequency Ω 0, the frequency uncertainty caused by the scalar Stark shift and second-order Doppler shift induced by micromotion is reduced to the 10−19 level. By precisely measuring the differential static scalar polarizability Δα 0, the uncertainty due to the blackbody radiation (BBR) shift (coefficient) is reduced to the 10−19 level. With the help of a second-order integrating servo algorithm, the uncertainty due to the servo error is reduced to the 10−18 level. The total fractional uncertainty of the 40Ca+ ion optical clock is then improved to 2.2× 10−17, whereas this value is mainly restricted by the uncertainty of the BBR shift due to temperature fluctuations. The state preparation is introduced together with improvements in the pulse sequence, and furthermore, a better signal to noise ratio (SNR) and less dead time are achieved. The clock stability of a single clock is improved to 4.8 × 10 − 15 / τ ( in seconds ) .