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European Frequency and Time Forum

Stability Improvement of an Aluminum Ion Optical Clock

B. Wang, Z. Y. Ma, Liren Pang, R. Gong, S. Q. Wei, Ke Deng, J. Zhang, Z. H. Lu

Published Jun 25, 2024• 0 citations
Publisher

Abstract

Based on the current initiative to redefine the SI “second”, the optical clock has been demonstrated by many laboratories as a favorable candidate due to its excellent performance indicators. We describe the evaluation of uncertainty and optimization of stability in an aluminum-ion optical clock system based on quantum logic spectroscopy. The uncertainty evaluation of the current optical clock system is$1.6\times 10^{-18}$. The frequency stability of a single system has been improved from 3.9$\times 10^{-15} / \sqrt{\tau}$to 2.6$\times 10^{-15} / \sqrt{\tau}$by using the servo-controlled self-comparison locking method.

DOI
10.1109/eftf61992.2024.10722614

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Optical clocks have been demonstrated to be good choices to redefine the SI ``second.'' With this goal, we have constructed a quantum-logic-based 27{}^{27}27Al\mathrm{Al}Al+{}^{+}+ ion optical clock with a fractional frequency uncertainty of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$. A 25{}^{25}25Mg\mathrm{Mg}Mg+{}^{+}+ ion is used as the logic ion for sympathetic cooling, state readout, and frequency-shift measurement of the 27{}^{27}27Al\mathrm{Al}Al+{}^{+}+ ion. The stability of the optical clock is $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}/\sqrt{\ensuremath{\tau}}$, measured with a self-comparison method.
Direct Laser Cooling Al+{}^{+}+ Ion Optical Clocks
The Al ion optical clock is a very promising optical frequency standard candidate due to its extremely small black-body radiation shift. It has been successfully demonstrated with the indirect cooled, quantum-logic-based spectroscopy technique. Its accuracy is limited by second-order Doppler shift, and its stability is limited by the number of ions that can be probed in quantum logic processing. We propose a direct laser cooling scheme of Al ion optical clocks where both the stability and accuracy of the clocks are greatly improved. In the proposed scheme, two Al traps are utilized. The first trap is used to trap a large number of Al ions to improve the stability of the clock laser, while the second trap is used to trap a single Al ion to provide the ultimate accuracy. Both traps are cooled with a continuous wave 167 nm laser. The expected clock laser stability can reach . For the second trap, in addition to 167 nm laser Doppler cooling, a second stage pulsed 234 nm two-photon cooling laser is utilized to further improve the accuracy of the clock laser. The total systematic uncertainty can be reduced to about . The proposed Al ion optical clock has the potential to become the most accurate and stable optical clock .
We report a single-ion optical atomic clock with a fractional frequency uncertainty of 5.5×10^{-19} and fractional frequency stability of 3.5×10^{-16}/sqrt[τ/s], based on quantum logic spectroscopy of a single ^{27}Al^{+} ion. A cotrapped ^{25}Mg^{+} ion provides sympathetic cooling and quantum logic readout of the ^{27}Al^{+}^{1}S_{0}↔^{3}P_{0} clock transition. A Rabi probe duration of 1 s, enabled by laser stability transfer from a remote cryogenic silicon cavity across a 3.6 km fiber link, results in a threefold reduction in instability compared to previous ^{27}Al^{+} clocks. Systematic uncertainties are lower due to an improved ion trap electrical design, which reduces excess micromotion, and a new vacuum system, which reduces collisional shifts. We also perform a direction-sensitive measurement of the ac magnetic field due to the rf ion trap, eliminating systematic uncertainty due to field orientation.
This paper presents an overview of the Al+ optical clock, which is based on quantum logic spectroscopy and provides the most accurate frequency measurement ever reported. Dehmelt’s original design of the optical clock was not realized due to the difficulty in generating the vacuum ultraviolet radiation necessary for laser cooling and the ion’s quantum state detection. The problem was solved by sympathetically cooling the ion with another ion and detecting the quantum state using the ancillary ion. The method, which is called quantum logic spectroscopy, comes from the use of phonon qubit as a bus of the quantum state of the ions in the Cirac-Zoller ion-trap quantum computer. The clock can be simplifi ed by introducing direct excitation by vacuum ultraviolet radiation or another ion species. The clock’s stability will be improved more than ten times by a middle-term stabilized clock laser.
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We have constructed an optical clock with a fractional frequency inaccuracy of 8.6x10{-18}, based on quantum logic spectroscopy of an Al+ ion. A simultaneously trapped Mg+ ion serves to sympathetically laser cool the Al+ ion and detect its quantum state. The frequency of the {1}S{0}<-->{3}P{0} clock transition is compared to that of a previously constructed Al+ optical clock with a statistical measurement uncertainty of 7.0x10{-18}. The two clocks exhibit a relative stability of 2.8x10{-15}tau{-1/2}, and a fractional frequency difference of -1.8x10{-17}, consistent with the accuracy limit of the older clock.