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

A new trapped ion clock based on 201HG+

S. Taghavi-Larigani, Eric A. Burt, S.N. Lea, John D. Prestage, Robert L. Tjoelker

Published Apr 1, 2009• 0 citations
Publisher

Abstract

There are two stable odd isotopes of mercury with singly ionized hyperfine structure suitable for a microwave clock:199Hg+and201Hg+. Virtually all trapped mercury ion clocks to date have used the 199 isotope. We have begun to investigate the viability of a trapped ion clock based on201Hg+. We have measured the unperturbed frequency of the2S1/2F = 1, mF= 0 to2S1/2F = 2, mF=0 clock transition to be 29.9543658211(2) GHz. In this paper we describe initial measurements with201Hg+and new applications to clocks and fundamental physics.

DOI
10.1109/freq.2009.5168290

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There are two stable odd isotopes of mercury with singly ionized hyperfine structure suitable for a microwave clock:199Hg+and201Hg+. Virtually all trapped mercury ion clocks to date have used the 199 isotope. We have begun to investigate the viability of a trapped ion clock based on201Hg+. We have measured the unperturbed frequency of the2S1/2F = 1, mF= 0 to2S1/2F = 2, mF=0 clock transition to be 29.9543658211(2) GHz. In this paper we describe initial measurements with201Hg+and new applications to clocks and fundamental physics.
As part of an on-going effort to build a dual-isotope ion trap clock based on the199Hg+and201Hg+ground state hyperfine transitions, we continue to characterize the systematic sensitivities of the201Hg+29.9GHz clock transition. Here we present an extension to the previous201Hg+collision shift measurement for helium[1] to other background gases. We also measured the collision shifts in the199Hg+40.5GHz clock transition in the same physical system. We find that frequency sensitivities to collisions with background gas for the two isotopes agree within a factor of two. We will also discuss planned collision shift measurements with co-trapped199Hg+and201Hg+, as an important step towards a dual-isotope atomic clock.
Trapped mercury ion is a promising candidate for portable and operational atomic clocks. It operates at the 40.5GHz ground state hyperfine transition of 199Hg ions. Mercury ions are confined in a linear RF trap, buffer-gas cooled, with a long ion storage lifetime in an ultra-high vacuum enclosure without an active pump. The clock transition is optically-pumped and detected with a Hg discharge lamp without need of a laser light source. This approach has proven to provide simplicity in design and implementation and robustness in operation. In this paper, we report our recently effort on developing a micro and low-power mercury trapped ion clock in the DARPA ACES program. In a 5 mm x 10 mm linear trap (0.25 cc), we have achieved the clock fractional frequency stability of .7 × 10-12τ-1/2and reached down to 7 × 10-14after 1000 seconds of averaging. We will discuss the clock performance under different conditions and implications toward a micro mercury trapped ion clock with low power and high stability.
In this paper, we report our recent effort on developing a micro mercury trapped ion atomic clock. In particular, we present preliminary results of a mercury-ion clock using a 15-cc vacuum trap tube. The mercury-ion clock can achieve a short-term fractional frequency instability of 1.5 ×10!12τ!1/2and reach a fractional frequency instability of 3 ×10!14after averaging for an hour. The clock can remain at the 10!14fractional frequency instability level up to several hours. Furthermore, the sealed 15-cc vacuum tube has demonstrated a very long ion lifetime of over two hundred days. We also investigated the mercury vapor density inside the vacuum trap tube by measuring the coherence time of the microwave clock transition.
Microwave atomic clocks have been the de facto standards for precision time and frequency metrology over the past 50 years, finding widespread use in basic scientific studies, communications, and navigation. However, with its higher operating frequency, an atomic clock based on an optical transition can be much more stable. We demonstrate an all-optical atomic clock referenced to the 1.064-petahertz transition of a single trapped 199Hg+ ion. A clockwork based on a mode-locked femtosecond laser provides output pulses at a 1-gigahertz rate that are phase-coherently locked to the optical frequency. By comparison to a laser-cooled calcium optical standard, an upper limit for the fractional frequency instability of 7 x 10(-15) is measured in 1 second of averaging-a value substantially better than that of the world's best microwave atomic clocks.
Local magnetic field strength in a trapped ion atomic clock is measured in real time, with high accuracy and without degrading clock performance, and the measurement is used to compensate for ambient magnetic field perturbations. First and second isotopes of an element are co-located within the linear ion trap. The first isotope has a resonant microwave transition between two hyperfine energy states, and the second isotope has a resonant Zeeman transition. Optical sources emit ultraviolet light that optically pump both isotopes. A microwave radiation source simultaneously emits microwave fields resonant with the first isotope's clock transition and the second isotope's Zeeman transition, and an optical detector measures the fluorescence from optically pumping both isotopes. The second isotope's Zeeman transition provides the measure of magnetic field strength, and the measurement is used to compensate the first isotope's clock transition or to adjust the applied C-field to reduce the effects of ambient magnetic field perturbations.
We report measured buffer gas collision shifts of the 40.507347996xx GHz mercury ion clock transition using inert helium, neon, and argon gases and getterable molecular hydrogen and nitrogen gases. The frequency shift due to methane was also examined. At low partial pressures methane gas did not impact the trapped ion number but was observed to strongly relax the population difference in the two hyperfine clock states thereby reducing the clock resonance signal. A similar population relaxation was also observed for other molecular buffer gases (N/sub 2/, H/sub 2/) but at much reduced rate.