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Physical Review Applied

Quantum-logic-based 25 Mg + - 27 Al + optical frequency standard for the redefinition of the SI second

Zhiyu Ma, Ke Deng, Z. Y. Wang, W. Z. Wei, P. Hao, H. X. Zhang, Liren Pang, B. Wang, Fang-Fei Wu, H. L. Liu, W. H. Yuan, J. L. Chang, J. X. Zhang, Quanying Wu, Jie Zhang, Z. H. Lu

Published Apr 8, 2024• 17 citations
Publisher

Abstract

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}$$\mathrm{Al}$${}^{+}$ ion optical clock with a fractional frequency uncertainty of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$. A ${}^{25}$$\mathrm{Mg}$${}^{+}$ ion is used as the logic ion for sympathetic cooling, state readout, and frequency-shift measurement of the ${}^{27}$$\mathrm{Al}$${}^{+}$ 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.

DOI
10.1103/physrevapplied.21.044017

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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}$$\mathrm{Al}$${}^{+} ion optical clock with a fractional frequency uncertainty of 1.6\ifmmode×\else\texttimes\fi10\ensuremath−181.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}1.6\ifmmode×\else\texttimes\fi10\ensuremath−18. A {}^{25}$$\mathrm{Mg}$${}^{+} ion is used as the logic ion for sympathetic cooling, state readout, and frequency-shift measurement of the {}^{27}$$\mathrm{Al}$${}^{+} ion. The stability of the optical clock is 2.6\ifmmode×\else\texttimes\fi10\ensuremath−15/\ensuremathτ2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}/\sqrt{\ensuremath{\tau}}2.6\ifmmode×\else\texttimes\fi10\ensuremath−15/\ensuremathτ​, measured with a self-comparison method.
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.
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.
We describe an optical atomic clock based on quantum-logic spectroscopy of the ^{1}S_{0}↔^{3}P_{0} transition in ^{27}Al^{+} with a systematic uncertainty of 9.4×10^{-19} and a frequency stability of 1.2×10^{-15}/sqrt[τ]. A ^{25}Mg^{+} ion is simultaneously trapped with the ^{27}Al^{+} ion and used for sympathetic cooling and state readout. Improvements in a new trap have led to reduced secular motion heating, compared to previous ^{27}Al^{+} clocks, enabling clock operation with ion secular motion near the three-dimensional ground state. Operating the clock with a lower trap drive frequency has reduced excess micromotion compared to previous ^{27}Al^{+} clocks. Both of these improvements have led to a reduced time-dilation shift uncertainty. Other systematic uncertainties including those due to blackbody radiation and the second-order Zeeman effect have also been reduced.
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 is1.6×10−181.6\times 10^{-18}1.6×10−18. The frequency stability of a single system has been improved from 3.9×10−15/τ\times 10^{-15} / \sqrt{\tau}×10−15/τ​to 2.6×10−15/τ\times 10^{-15} / \sqrt{\tau}×10−15/τ​by using the servo-controlled self-comparison locking method.
With the advent of optical clocks featuring fractional frequency uncertainties on the order of 10-17 and below, new applications such as chronometric leveling with few-centimeter height resolution emerge. We are developing a transportable optical clock based on a single trapped aluminum ion, which is interrogated via quantum logic spectroscopy. We employ singly charged calcium as the logic ion for sympathetic cooling, state preparation, and readout. Here, we present a simple and compact physics and laser package for manipulation of 40Ca+. Important features are a segmented multilayer trap with separate loading and probing zones, a compact titanium vacuum chamber, a near-diffraction-limited imaging system with high numerical aperture based on a single biaspheric lens, and an all-in-fiber 40Ca+ repump laser system. We present preliminary estimates of the trap-induced frequency shifts on 27Al+, derived from measurements with a single calcium ion. The micromotion-induced second-order Doppler shift for 27Al+ has been determined to be δνEMMν=-0.4-0.3 +0.4×10-18 and the black-body radiation shift is δνBBR/ν = (-4.0 ± 0.4) × 10-18. Moreover, heating rates of 30 (7) quanta per second at trap frequencies of ωrad,Ca+ ≈ 2π × 2.5 MHz (ωax,Ca+ ≈ 2π × 1.5 MHz) in radial (axial) direction have been measured, enabling interrogation times of a few hundreds of milliseconds.
With the advent of optical clocks featuring fractional frequency uncertainties on the order of 10−17 and below, new applications such as chronometric leveling with few-centimeter height resolution emerge. We are developing a transportable optical clock based on a single trapped aluminum ion, which is interrogated via quantum logic spectroscopy. We employ singly charged calcium as the logic ion for sympathetic cooling, state preparation, and readout. Here, we present a simple and compact physics and laser package for manipulation of 40Ca+. Important features are a segmented multilayer trap with separate loading and probing zones, a compact titanium vacuum chamber, a near-diffraction-limited imaging system with high numerical aperture based on a single biaspheric lens, and an all-in-fiber 40Ca+ repump laser system. We present preliminary estimates of the trap-induced frequency shifts on 27Al+, derived from measurements with a single calcium ion. The micromotion-induced second-order Doppler shift for 27Al+ has been determined to be δνEMMν=−0.4−0.3+0.4×10−18 and the black-body radiation shift is δνBBR/ν = (−4.0 ± 0.4) × 10−18. Moreover, heating rates of 30 (7) quanta per second at trap frequencies of ωrad,Ca+ ≈ 2π × 2.5 MHz (ωax,Ca+ ≈ 2π × 1.5 MHz) in radial (axial) direction have been measured, enabling interrogation times of a few hundreds of milliseconds.