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CAOL . International Conference on Advanced Optoelectronics and Lasers

Single ion spectroscopy for optical clocks

Chr. Tamm, E. Peik, T. Schneider, Harald R. Telle, J. Stenger, B. Lipphardt, H. Schnatz, Carl O. Weiss

Published Jul 8, 2004• 1 citation
Publisher

Abstract

The 435 mum171Yb+transition is a good candidate for an atomic clock based on an optical transition. We have stored single171Yb ions for several months in RF traps. The ions are cooled, repumped and the clock transition is interrogated by (frequency-doubled) diode lasers. The interrogation laser linewidth was 30 Hz determining the resolvable linewidth of the Yb+transition interrogated. The absolute frequency of this transition was measured using an optical comb generator, self referenced by broadening the spectrum to above one octave, with respect to a Cs fountain clock. Of all systematic frequency shifts to which this transition is subject an estimate indicates that the electric quadruple shift appears is the largest. To obtain a magnitude of the quadruple shift (and the quadruple moment) the clock frequencies of two independently stored and cooled171Yb ions were compared while varying the static electric field in one trap. The measured frequency shift amounting to ~1 Hz/V. This result would seem to permit clocks with much higher accuracy than the clocks based on microwave transition, particularly if the technique of aligning the ions with respect to the electric field were applicable. Anticipating even more precise clocks, we are presently investigating a blue nuclear transition in229Th. Such a nuclear transition should be even better shielded from environmental influences than transitions in the electron shell. This nuclear transition has not yet been directly optically observed. We are presently investigating the luminescence after incoherent broad band optical excitation. The triply ionized223Th is then a suitable system for trapped single ion spectroscopy. The cooling transition lies conveniently in the near infrared and the nuclear clock transition can be read out by nuclear/electron shell double resonance

DOI
10.1109/caol.2003.1251328

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The 435 mum171Yb+transition is a good candidate for an atomic clock based on an optical transition. We have stored single171Yb ions for several months in RF traps. The ions are cooled, repumped and the clock transition is interrogated by (frequency-doubled) diode lasers. The interrogation laser linewidth was 30 Hz determining the resolvable linewidth of the Yb+transition interrogated. The absolute frequency of this transition was measured using an optical comb generator, self referenced by broadening the spectrum to above one octave, with respect to a Cs fountain clock. Of all systematic frequency shifts to which this transition is subject an estimate indicates that the electric quadruple shift appears is the largest. To obtain a magnitude of the quadruple shift (and the quadruple moment) the clock frequencies of two independently stored and cooled171Yb ions were compared while varying the static electric field in one trap. The measured frequency shift amounting to ~1 Hz/V. This result would seem to permit clocks with much higher accuracy than the clocks based on microwave transition, particularly if the technique of aligning the ions with respect to the electric field were applicable. Anticipating even more precise clocks, we are presently investigating a blue nuclear transition in229Th. Such a nuclear transition should be even better shielded from environmental influences than transitions in the electron shell. This nuclear transition has not yet been directly optically observed. We are presently investigating the luminescence after incoherent broad band optical excitation. The triply ionized223Th is then a suitable system for trapped single ion spectroscopy. The cooling transition lies conveniently in the near infrared and the nuclear clock transition can be read out by nuclear/electron shell double resonance
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.
The171Yb+ion possesses two transitions that are applicable as references for optical frequency standards and several general features make this ion attractive: All relevant radiations for cooling and excitation can be generated from diode lasers. A nuclear spin of 1/2 ensures a simple sublevel structure. Long storage times of Yb+in ion traps are easily obtained. The2S1/2(F = 0) →2D3/2(F = 2, mF= 0) electric quadrupole transition at 436 nm with a natural linewidth of 3.1 Hz provides a quantum noise limited instability of a single-ion frequency standard of about 2 × 10-15in 1 s. The electric octupole transition2S1/2(F = 0)→2F7/2(F = 3, mF= 0) at 467 nm with a natural linewidth in the nHz range will allow to surpass that instability level. It also possesses a lower sensitivity to field-induced frequency shifts. In this system the Fourier-limited interrogation linewidth can be adapted to that of the best available laser sources, which are presently at the level below 1 Hz. Both transitions have been recommended as secondary representations of the SI second.
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.
We describe experimental investigations on an optical frequency standard based on a single laser cooled 171Yb+ ion confined in a radiofrequency Paul trap. The 435.5 nm electric-quadrupole transition from the 2S1/2(F=0) groudn state to the 2D3/2(F=2) state is probed using a diode laser system whose short-term frequency stability is derived from an environmentally isolated ULE cavity. The transition is resolved with a Foruier-limited full halfwidth of approximately 30 Hz. The absolute optical transition frequency is determined using a femtosecond frequency comb generator and a Cs fountain clock as the frequency reference. The measured frequency value is 688 358 979 321 Hz at an ambient blackbody radiation field of 300 K. The 1σ fractional uncertainty of this frequency measurement is 1×10-14.
The National Institute of Information and Communications Technology in Japan is proceeding with development of an optical frequency standard using an electric quadrupole transition of Ca+ ions in a small radio-frequency trap. The single, trapped 40Ca+ ions are laser cooled to a temperature of <10 mK using extended-cavity diode lasers at 397 nm and 866 nm. In order to measure the electric quadrupole transition, the linewidth of the clock laser diode at 729 nm is reduced to ~66 Hz using an ultra-low-expansion (ULE) cavity. We observed the electric quadrupole transition with a linewidth of ~600 kHz, together with the ionic motional sidebands. We are also developing optical frequency combs without photonic fibers broadening the spectrum. The experimental is now improved in order to observe the narrower linewidth of the quadrupole transition.
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.