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Chinese Physics Letters

Long-Lifetime Optical Trapping of a 40Ca+ Ion

Z. L. Chen, Miao 淼 Wang 王, Baolin Zhang, Huaqing Zhang, Zixiao 子晓 Ma 马, Ruming Hu, Yao Huang, Kelin Gao, Hua Guan

Published Sep 27, 2024• 0 citations
Publisher

Abstract

Abstract We have experimentally achieved the all-optical trapping of a 40Ca+ ion. An optical dipole trap was established using a high-power, far-detuned, tightly focused laser with a wavelength of 532 nm. The single 40Ca+ ion was trapped without any RF fields and demonstrated a long lifetime of over 3 s. In this experiment, we implemented several measures to improve the optical trapping probability, including focusing the dipole beam waist near the diffraction limit, precisely compensating for stray electric fields, and mitigating electron shelving in metastable states. The optical trapping of a 40Ca+ ion eliminates the influence of micromotion induced by RF fields, potentially paving the way for development of all-optical trapping ion optical clocks.

DOI
10.1088/0256-307x/41/11/113701

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Abstract We have experimentally achieved the all-optical trapping of a 40Ca+ ion. An optical dipole trap was established using a high-power, far-detuned, tightly focused laser with a wavelength of 532 nm. The single 40Ca+ ion was trapped without any RF fields and demonstrated a long lifetime of over 3 s. In this experiment, we implemented several measures to improve the optical trapping probability, including focusing the dipole beam waist near the diffraction limit, precisely compensating for stray electric fields, and mitigating electron shelving in metastable states. The optical trapping of a 40Ca+ ion eliminates the influence of micromotion induced by RF fields, potentially paving the way for development of all-optical trapping ion optical clocks.
Micromotion induced by the radio-frequency field contributes greatly to the systematic frequency shifts of optical frequency standards. Although different strategies for mitigating this effect have been proposed, trapping ions optically has the potential to provide a generic solution to the elimination of micromotion. This could be achieved by trapping a single ion in the dipole trap composed of a highpower laser field. Here, we present the setup of the dipole trap composed of a 532 nm laser at a power of 10 W aiming to optically trap a single 40Ca+ and we observe an AC-Stark shift of the fluorescence spectrum line of ∼22 MHz caused by the 532 nm dipole beam. The beam waist of the dipole laser is several microns, which would provide a dipole potential strong enough for all-optical trapping of a single 40Ca+ ion.
Abstract For the first time, we experimentally determine the infrared magic wavelength for the 40Ca+ 4 s 2 S 1 / 2 → 3 d 2 D 5 / 2 electric quadrupole transition by observation of the light shift canceling in 40Ca+ optical clock. A ‘magic’ magnetic field direction is chosen to make the magic wavelength insensitive to both the linear polarization purity and the polarization direction of the laser. The determined magic wavelength for this transition is 1056.37(9) nm, which is not only in good agreement with theoretical predictions (‘Dirac–Fock plus core polarization’ method) but also more precise by a factor of about 300. Using this measured magic wavelength, we also derive the differential static polarizability to be −44.32(32) a.u., which will be an important input for the evaluation of the blackbody radiation shift at room temperatures. Our work paves a way for all-optical-trapping of 40Ca+ optical clock.
Experimental research toward the realization of an optical frequency standard of high accuracy is briefly reviewed. The studies concentrated on single, laser-cooled ions since they offer several attractive features for the achievement of high accuracy. These features include long storage time which eliminates transit-time broadening, confinement that is nearly nonperturbative to the internal level structure of the ion, laser-cooling that reduces motional shifts to small values, and electron-shelving whereby transitions to long-lived states can be detected with unit probability. The authors studied spectroscopically the electric-quadrupole allowed transition at 282 nm in a single, laser-cooled /sup 199/Hg/sup +/ ion stored in an RF Paul trap with extremely high resolution. The measured linewidth is limited by the spectral purity of the laser to about 80 Hz. Possible improvements and future directions are discussed.>
The concept of an active optical frequency standard was proposed about 10 years ago. The idea was to use optically trapped alkaline-earth atoms as a gain medium to built an extremely narrow-line laser, whose frequency would be robust against fluctuations of the cavity length. The main challenge in the realization of this concept is the short trap lifetime of the atoms. Recently we showed, that in such a laser, neutral atoms may be replaced by charged ions in a radio-frequency (RF) Paul trap with much longer lifetime. This idea rests on two pillars: the effect of synchronization of radiating dipoles with moderate inhomogeneous broadening, and the possibility to compensate (in leading orders) micromotion-induced shifts for some ion species in specially designed traps.
We report on single Barium ions confined in a near-infrared optical dipole trap for up to three seconds in absence of any radio-frequency fields. Additionally, the lifetime in a visible optical dipole trap is increased by two orders of magnitude as compared to the state-of-the-art using an efficient repumping method. We characterize the state-dependent potentials and measure an upper bound for the heating rate in the near-infrared trap. These findings are beneficial for entering the regime of ultracold interaction in atom-ion ensembles exploiting bichromatic optical dipole traps. Long lifetimes and low scattering rates are essential to reach long coherence times for quantum simulations in optical lattices employing many ions, or ions and atoms.
We recently reported on a proof-of-principle experiment demonstrating optical trapping of an ion in a single-beam dipole trap superimposed by a static electric potential [Ch. Schneider, M. Enderlein, T. Huber, and T. Schaetz, Nat. Photon. 4, 772 (2010)]. Here, we discuss, first, the experimental procedures focusing on the influence and consequences of the static electric potential. This potential can easily prevent successful optical trapping, if its configuration is not chosen carefully. Afterward, we analyze the dipole trap experiments with different analytic models, in which different approximations are applied. According to these models the experimental results agree with recoil heating as the relevant heating effect. In addition, a Monte Carlo simulation has been developed to refine the analysis. It reveals a large impact of the static electric potential on the dipole trap experiments in general. While it supports the results of the analytic models for the parameters used in the experiments, the analytic models are no longer valid for significantly different parameters. Finally, we propose technical improvements for future realizations of experiments with optically trapped ions.