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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Design of a Low Uncertainty Ion Trap for a Transportable Ion Optical Clock

P. Dubé

Published Apr 1, 2019• 0 citations
Publisher

Abstract

We are designing an rf trap for a transportable optical clock based on the strontium ion. This new design aims to minimize systematic shifts, especially the blackbody radiation shift, and to optimize the geometry to reduce the electric quadrupole shift and provide efficient laser cooling along the three principal axes of ion motion. This new trap will be part of a transportable optical clock that will have an uncertainty expected to reach the low 10-18level.

DOI
10.1109/fcs.2019.8855992

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We are designing an rf trap for a transportable optical clock based on the strontium ion. This new design aims to minimize systematic shifts, especially the blackbody radiation shift, and to optimize the geometry to reduce the electric quadrupole shift and provide efficient laser cooling along the three principal axes of ion motion. This new trap will be part of a transportable optical clock that will have an uncertainty expected to reach the low 10-18level.
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.
The uncertainty of the ac Stark shift due to thermal radiation represents a major contribution to the systematic uncertainty budget of state-of-the-art optical atomic clocks. In the case of optical clocks based on trapped ions, the thermal behavior of the rf-driven ion trap must be precisely known. This determination is even more difficult when scalable linear ion traps are used. Such traps enable a more advanced control of multiple ions and have become a platform for new applications in quantum metrology, simulation, and computation. Nevertheless, their complex structure makes it more difficult to precisely determine its temperature in operation and thus the related systematic uncertainty. We present here scalable linear ion traps for optical clocks, which exhibit very low temperature rise under operation. We use a finite-element model refined with experimental measurements to determine the thermal distribution in the ion trap and the temperature at the position of the ions. The trap temperature is investigated at different rf-drive frequencies and amplitudes with an infrared camera and integrated temperature sensors. We show that for typical trapping parameters for In+, Al+, Lu+, Ca+, Sr+, or Yb+ ions, the temperature rise at the position of the ions resulting from rf heating of the trap stays below 700 mK and can be controlled with an uncertainty on the order of a few 100 mK maximum. The corresponding uncertainty of the trap-related blackbody radiation shift is in the low 10−19 and even 10−20 regime for 171Yb+(E3) and 115In+, respectively.
A liquid-nitrogen cryogenic 40Ca+ optical clock is presented that is designed to greatly reduce the blackbody radiation (BBR) shift. The ion trap, the electrodes and the in-vacuum BBR shield are installed under the liquid-nitrogen container, keeping the ions in a cryogenic environment at liquid-nitrogen temperature. Compared with the first design in our previous work, many improvements have been made to increase the performance. The liquid-nitrogen maintenance time has been increased by about three times by increasing the volume of the liquid-nitrogen container; the trap position recovery time after refilling the liquid-nitrogen container has been decreased more than three times by using a better fixation scheme in the liquid-nitrogen container; and the magnetic field noise felt by the ions has been decreased more than three times by a better design of the magnetic shielding system. These optimizations make the scheme for reducing the BBR shift uncertainty of liquid-nitrogen-cooled optical clocks more mature and stable, and develop a stable lock with a narrower linewidth spectrum, which would be very beneficial for further reducing the overall systematic uncertainty of optical clocks.
Optical atomic clocks demonstrate a better stability and lower systematic uncertainty than the highest performance microwave atomic clocks. However, the best performing optical clocks have a large footprint in a laboratory environment and require specialist skills to maintain continuous operation. Growing and evolving needs across several sectors are increasing the demand for compact robust and portable devices at this capability level. In this paper we discuss the design of a physics package for a compact laser-cooled 88 Sr + optical clock that would, with further development, be suitable for space deployment. We review the design parameters to target a relative frequency uncertainty at the low parts in 10 18 with this system. We then explain the results of finite-element modelling to simulate the response of the ion trap and vacuum chamber to vibration, shock and thermal conditions expected during launch and space deployment. Additionally, an electrostatic model has been developed to investigate the relationship between the ion trap geometrical tolerances and the trapping efficiency. We present the results from these analyses that have led to the design of a more robust prototype ready for experimental testing.
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.