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Conference on Precision Electromagnetic Measurements

Progress toward a new implementation of an In+optical clock

Ying Li, Nozomi Ohtsubo, Kensuke Matsubara, T. Ido, Kazuhiro Hayasaka

Published Jul 1, 2016• 0 citations
Publisher

Abstract

Recent progress in implementing an In+optical clock based on an In+sympathetically-cooled with Ca+is reported. Our new implementation is expected to achieve a frequency uncertainty bellow the order of 10−17as well as a future extension to a multi-ion optical clock with enhanced stability. Direct detection of an In+in a linear trap with Lamb-Dicke confinement using the1S0−3P1transition at 230 nm is successfully demonstrated, which enables the clock operation of the system.

DOI
10.1109/cpem.2016.7540550

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Recent progress in implementing an In+optical clock based on an In+sympathetically-cooled with Ca+is reported. Our new implementation is expected to achieve a frequency uncertainty bellow the order of 10−17as well as a future extension to a multi-ion optical clock with enhanced stability. Direct detection of an In+in a linear trap with Lamb-Dicke confinement using the1S0−3P1transition at 230 nm is successfully demonstrated, which enables the clock operation of the system.
We report on the progress toward an indium ion optical frequency standard with an expected accuracy of 10-18. A115In+has been successfully prepared by sympathetic cooling with two laser-cooled40Ca+ions. The clock laser at 237 nm has been constructed as the forth harmonic of an amplified diode laser with a linewidth smaller than 0.9 Hz. After beam shaping and intensity stabilization a power over 10 mW is available for the clock transition (1S0-3P0) interrogation. Observation of the clock transition is in progress by employing the electron shelving method using the1S0-3P1transition at 230 nm.
The optical clock is a fundamental applications of the coherence of ultracold atoms. Most of the implementations with trapped ions are based on the single-ion optical clock by Dehmelt [1], and some of them have successfully achieved inaccuracies in the 10-18level [2]. The idea has been extended to systems consisting of many neutral atoms by Katori with proper engineering on light shift associated with the optical lattice [3]. The enhanced stability is the great advantage obtained by multiple atoms. Extension of the single-ion system to multi-ion systems requires considerations on the inhomogeneous electric quadrupole shift originating from non-uniform electric field gradient at the location of each ion. A straightforward approach is to use ion species with null quadrupole moment such as In+, Al+[4]. We present another approach using ions with non-zero quadrupole moment. In the approach we use a planar surface trap to arrange the ions in an equally-spaced manner to vanish the inhomogeneous shift. We show that a clock with multiple Ca+ions can be operated even without a reference cavity for the clock laser, and that superradiant clock is expected when the ions are coupled to a cavity.
An optical frequency standard based on the $S\text{\ensuremath{-}}D$ transition of trapped 43Ca+^{43}\mathrm{Ca}^{+}43Ca+ is possible by using just laser diodes for cooling and probing. We discuss two open problems related to the feasibility of this frequency standard, the attainable frequency uncertainty and the method of initial-state preparation. The uncertainty of the clock transition frequency can be so small as $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$. Using a laser system consisting of three lasers at different detunings and polarizations, the ion can be cooled down to the Lamb-Dicke regime and then prepared in the m=0m=0m=0 state within a period much shorter than the cooling period without being heated. The details of our numerical simulation are described.
An ultra-stable optical clock based on coherent population trapping effect of alkaline-earth ions, such as Ca+^++, Sr+^++, Ba+^++, is analyzed here. The proposed transitions use the odd isotopes, so that the frequency shift is insensitive to the applied magnetic field. To enhance the signal, a large number of ions are trapped in a linear Paul trap, and laser cooled to crystallize such that the ions are in the Lamb-Dicke regime to avoid first order Doppler shift. Other relevant frequency shifts are also analyzed. Further techniques to improve the stability of the clock signal are discussed.
For the past 15 years, tremendous progress within the fields of laser stabilization, optical frequency combs and atom cooling and trapping have allowed the realization of optical atomic clocks with unrivaled performances. These instruments can perform frequency comparisons with fractional uncertainties well below 10-17, finding applications in fundamental physics tests, relativistic geodesy and time and frequency metrology. Even though most optical clocks are currently laboratory setups, several proposals for using these clocks for field measurements or within an optical clock network have been published, and most of time and frequency metrology institutes have started to develop transportable optical clocks. For the purpose of this special issue, we chose to focus on trapped-ion optical clocks. Even though their short-term fractional frequency stability is impaired by a lower signal-to-noise ratio, they offer a high potential for compactness: trapped ions demand low optical powers and simple loading schemes, and can be trapped in small vacuum chambers. We review recent advances on the clock key components, including ion trap and ultra-stable optical cavity, as well as existing projects and experiments which draw the picture of what future transportable, single-ion optical clocks may resemble.