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Journal of Physics Conference Series

Recent progress on the27Al+ion optical clock

Zhilin Xu, W. H. Yuan, Xiangbin Zeng, H Che, Xiaohui Shi, Ke Deng, Jingxuan Zhang, Z. H. Lu

Published Jun 1, 2016• 8 citations
Publisher

Abstract

An aluminium ion optical clock is under development at Huazhong University of Science and Technology. The 25 Mg + ion is chosen as logic ion to sympathetically cool an Al + ion and to detect its states. The 25 Mg + ion is cooled to the motional ground state through Raman sideband cooling as the first step for quantum logic spectroscopy. Ultra-stable lasers for the interrogation of the clock transition are developed. The instability of the laser beat frequency is 1.2 x 10 -15 at 1 s, which is close to the thermal noise limit of the reference cavity.

DOI
10.1088/1742-6596/723/1/012026

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An aluminium ion optical clock is under development at Huazhong University of Science and Technology. The 25 Mg + ion is chosen as logic ion to sympathetically cool an Al + ion and to detect its states. The 25 Mg + ion is cooled to the motional ground state through Raman sideband cooling as the first step for quantum logic spectroscopy. Ultra-stable lasers for the interrogation of the clock transition are developed. The instability of the laser beat frequency is 1.2 x 10 -15 at 1 s, which is close to the thermal noise limit of the reference cavity.
Raman sideband cooling of 25^{25}25Mg+^++ -27^{27}27Al+^++ ions pair and observation of quantum logic spectra
We perform quantum logic spectroscopy (QLS) on 27^{27}27Al+^++ ion 1^11S0_00​ - 3^33P1_11​ transition, which is an important step toward the QLS based 27^{27}27Al+^++ ion optical clock. As a precondition of QLS, both the stretch (STR) mode and the common (COM) mode of the 27^{27}27Al+^++ and 25^{25}25Mg+^++ ions pair are cooled to the vibrational ground state by Raman sideband cooling. The mean phonon number is measured to be 0.10(1) for the STR mode and 0.01(1) for the COM mode, respectively. The heating rate is evaluated to be 13(3) phonons/s for the STR mode and 5.5(1) phonons/s for the COM mode.
Magnesium ion which acts as the logic ion in27Al+ion optical clock is trapped and laser cooled in a linear ion trap. Rabi oscillation between two hyperfine ground states of25Mg+ion is observed and resonant frequency between two hyperfine ground states of25Mg+ion is measured. Earth magnetic field is compensated with three pairs of mutually perpendicular Helmholtz coils. Light shift induced by the Raman cooling beams is measured. With the photo-ionization laser of Al, Al+ion is trapped together with Mg+ion. The linewidth of the clock laser of the27Al+ion is measured to be about 0.8 Hz. The Allan deviation of the beat note from two independent lasers is 6.5×10-15at 1s.
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.
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.
Research progress of Al+^{+}+ ion optical frequency standard based on quantum logic technique
Optical atomic clocks have many applications in the definition of the time and the length, measurement of fundamental constants and their potential variations with time, testing of relativity, very long baseline interferometry, geodesy, and global navigation satellite systems. An optical clock consists of three parts: ultra-stable laser, frequency comb and trapped ions or atoms. The accuracy of optical atomic clocks can be affected by Doppler shift, Zeeman shift, Stark shift, collision shift, blackbody radiation (BBR) shift, etc. The BBR shift is very difficult to be characterized and controlled. Among all optical atomic clocks under current development, Al+^++ optical clock has the smallest BBR shift at a level of 4.0×10−194.0\times10^{-19}4.0×10−19, therefore the Al+^++ is a promising ion for high-accuracy optical frequency standard. In this article we give a review on the three parts of optical frequency standards, the basic working principle of Al+^++ ion optical frequency standard based on quantum logic technique, and research progress of an Al+^++ optical clock under development at Huazhong University of Science and Technology.
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.