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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Towards an indium single-ion optical clock

E. Peik, J. Abel, Th. Becker, M. Fries, Joachim von Zanthier, HERBERT WALTHER

Published Jan 22, 2003• 1 citation
Publisher

Abstract

We are investigating the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 0/ transition of a single trapped laser-cooled /sup 115/In/sup +/ ion as a possible optical frequency standard. This line with a natural linewidth of only 1.1 Hz, corresponding to a Q factor of 1.2/spl middot/10/sup 15/, is highly immune to systematic frequency shifts due to external perturbations. For sideband laser cooling and fluorescence detection of the indium ion the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 1/ transition at 230.6 nm is excited. Temperatures below 100 /spl mu/K and a mean vibrational quantum number<1 of the ion in the trap have been reached. For the clock transition a resolution of 3/spl middot/10/sup -13/ (linewidth 350 Hz) has been obtained so far, limited by the laser linewidth. The absolute frequency of the /sup 1/S/sub 0//spl rarr//sup 3/P/sub 0/ transition was measured by making a link to known reference frequencies of molecular iodine and methane using a frequency chain.

DOI
10.1109/freq.1999.841397

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We are investigating the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 0/ transition of a single trapped laser-cooled /sup 115/In/sup +/ ion as a possible optical frequency standard. This line with a natural linewidth of only 1.1 Hz, corresponding to a Q factor of 1.2/spl middot/10/sup 15/, is highly immune to systematic frequency shifts due to external perturbations. For sideband laser cooling and fluorescence detection of the indium ion the 5s/sup 21/S/sub 0//spl rarr/5s5p/sup 3/P/sub 1/ transition at 230.6 nm is excited. Temperatures below 100 /spl mu/K and a mean vibrational quantum number<1 of the ion in the trap have been reached. For the clock transition a resolution of 3/spl middot/10/sup -13/ (linewidth 350 Hz) has been obtained so far, limited by the laser linewidth. The absolute frequency of the /sup 1/S/sub 0//spl rarr//sup 3/P/sub 0/ transition was measured by making a link to known reference frequencies of molecular iodine and methane using a frequency chain.
A single laser cooled ion in a radiofrequency trap can serve as the basis for a highly stable optical frequency standard. We present recent results of our work on single indium ions, using the /sup 1/S/sub 0/ to /sup 3/P/sub 0/ transition at 236.5 nm wave-length as the clock transition. This resonance has a linewidth of only 0.82 Hz where systematic shifts should be controllable on the mHz level. A single In/sup +/ ion is stored in a miniature Paul-Straubel trap and laser cooled to a temperature of about 100 /spl mu/K. The clock transition is excited using a frequency quadrupled Nd:YAG laser. A fractional resolution of 1.3/spl middot/10/sup -13/ has been achieved so far (linewidth of 170 Hz at 1267 THz). The absolute frequency of the clock transition clock transition has been measured with an uncertainty of 2/spl middot/10/sup -13/ using a frequency chain and a methane-stabilized HeNe laser as a reference, that was calibrated with a cesium clock.
Summary form only given. Laser-cooled ions in radiofrequency traps are the atomic systems that allow the highest resolution (ratio of frequency to linewidth) in optical or microwave spectroscopy. They can serve as references in highly stable frequency standards and also enable precise tests of fundamental physics. We present recent results on our In/sup +/ optical frequency standard, based on the 5s/sup 2/ /sup 1/S/sub 0//spl rarr/5s5p /sup 3/P/sub 0/ transition at 236.5 nm wavelength. This transition has a natural linewidth of only 0.82 Hz and is highly immune to systematic frequency shifts.
Single laser-cooled ions in radiofrequency traps can serve as the basis for highly stable optical frequency standards. Here we present recent results on In+, using the 1S0 yields 3P0 line at 236.5 nm wavelength as the clock transition. This resonance has a natural linewidth of only 0.82 Hz and systematic shifts should be controllable at the mHz level. A single indium ion is stored in a miniature Paul- Straubel trap and laser cooled to a temperature of about 100 (mu) K. The clock transition is excited using a frequency quadrupled 946 nm Nd:YAG laser. A fractional resolution of 1.3 (DOT) 10-13 has been achieved so far (linewidth of 170 Hz at 1267 THz). The absolute optical frequency of the clock transition has been measured with an uncertainty being less than 3 (DOT) 10-13 using a frequency chain and a methane-stabilized HeNe laser that was calibrated with a cesium clock.
In this paper experiments with single trapped ions are reviewed. They are performed with the aim of developing a new frequency standard and to perform cavity quantum electrodynamics experiments with single ions. For a new optical frequency standard we are investigating the 5s21S0 implied by 5s5p3P0 transition of a single trapped laser-cooled 115In(superscript + ion. This line with a natural linewidth of only 0.8 Hz is highly immune to systematic frequency shifts. For sideband laser cooling and fluorescence detection of the indium ion the 5s21S0 implied by 5s5p3P1 transition at 230.6 nm is used. Temperatures below 100 (mu) K and a mean vibrational quantum number <n><1 of the ion in the trap have been reached. For the clock transition a resolution of 1.3 multiplied by 10-13 (linewidth 170 Hz) has been obtained so far, limited by the short term frequency fluctuations of the clock laser. The absolute frequency of the 1S0 implied by 3P0 transition was measured by making a link to the reference frequency of the methane-stabilized HeNe laser using a frequency chain. In the second part of the paper the present status of the experiments on the ion trap laser are briefly described.
Single ions trapped and laser cooled provide ideal objects for high resolution spectroscopy and frequency standards. For the latter purpose we are investigating the 5s2 1S0 → 5s5p 3P0 transition of In+. With a natural linewidth of only 1.1 Hz this resonance offers very high resolution and is highly immune to frequency shifts due to external electromagnetic fields, because it connects two levels with vanishing electronic magnetic momenta [1]. The wavelength of this clock transition is 236.5 nm and is technically very convenient, since it coincides with the fourth harmonic of the 946 nm Nd:YAG laser line. So this intrinsically frequency stable solid-state laser can be used to excite the transition [2]. For laser cooling and fluorescence detection of the indium ion the 5s2 1So -> 5s5p 3P1 transition at 230.6 nm can be employed. The use of the relatively narrow intercombination line for laser cooling allows us to study optical sideband cooling in the strong binding regime, where the oscillation frequencies of the ion in the trap (around 1 MHz) are larger than the optical linewidth of 360 kHz. In this parameter range laser cooling is possible to the quantum ground state of the vibrational motion in the trap corresponding to a temperature of roughly 100 μK. The same low temperature was obtained with sideband cooling of a small Coulomb crystal consisting of two ions, creating an interesting new quantum few-particle system.
We report on the current status of the optical frequency standard based on a single 115In+ ion. A single Indium ion is being trapped in a Paul-Strauble type trap and cooled in combination of Doppler cooling and sideband cooling to its lowest vibrational states. To interrogate the1S0-3P0clock transition with high accuracy we demonstrate a Hz-level laser system resonant to this transition. With the current setup we are able to achieve a measured linewidth of the clock transition of 260 Hz. The statistical uncertainty of the absolute frequency measurement is 3×10-13, limited mainly by our reference frequency standard.