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Journal of Physics Conference Series

Quantum projection noise limited stability of a88Sr+ atomic clock

Bin Jian, P. Dubé, Alan A. Madej

Published Jun 1, 2016• 2 citations
Publisher

Abstract

The evaluated accuracy of a single trapped 88 Sr + ion clock referenced to the 5s 2 S 1/2 - 4d 2 D 5/2 transition at 445 THz at the National Research Council of Canada has reached 1.2 x 10 -17 over recent years. On the other hand, the stability of an atomic clock determines how long the signals from two similar clocks have to be compared to reach a given level of uncertainty. Here, we report on the improvement of the stability of NRC's 88 Sr + single ion clock by reducing the Allan deviation from 1 x 10 -14 to 3 x 10 -15 at 1 second averaging time. This is done by the implementation of a clear out laser that transfers the ion from the metastable state to the ground state at each cycle, followed by a state-preparation step that transfers the ion to the desired ground state magnetic sublevel of the probed transition.

DOI
10.1088/1742-6596/723/1/012023

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The evaluated accuracy of a single trapped 88 Sr + ion clock referenced to the 5s 2 S 1/2 - 4d 2 D 5/2 transition at 445 THz at the National Research Council of Canada has reached 1.2 x 10 -17 over recent years. On the other hand, the stability of an atomic clock determines how long the signals from two similar clocks have to be compared to reach a given level of uncertainty. Here, we report on the improvement of the stability of NRC's 88 Sr + single ion clock by reducing the Allan deviation from 1 x 10 -14 to 3 x 10 -15 at 1 second averaging time. This is done by the implementation of a clear out laser that transfers the ion from the metastable state to the ground state at each cycle, followed by a state-preparation step that transfers the ion to the desired ground state magnetic sublevel of the probed transition.
A number of optical frequency standards have been evaluated with fractional uncertainties between ${10}^{\ensuremath{-}17}$ and ${10}^{\ensuremath{-}18}$. Reduction of the statistical uncertainty of a clock comparison to this level is challenging, requiring the best possible stability to avoid excessively long averaging times. We report recent improvements of our 88Sr+^{88}\mathrm{Sr}{}^{+}88Sr+ single-ion standard that have reduced its 1-s Allan deviation from $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ to $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$, yielding an order of magnitude decrease in averaging time for a given statistical uncertainty level. Among the improvements made are the implementation of a clear-out laser that transfers the ion from the metastable state to the ground state at each cycle, followed by a state preparation step that transfers the ion to the ground-state magnetic sublevel of the probed transition. With these changes, the ion clock transition interacts with the probe laser at every interrogation cycle. The stability of our optical standard is essentially limited by the quantum projection noise for pulse lengths up to $\ensuremath{\approx}100$ ms.
We present the current status of the Sr+optical atomic ion clock at the National Research Council of Canada including the study of the quantum projection noise (QPN) limited stability and the progress on the evaluation of the blackbody radiation (BBR) shift uncertainty. The latest QPN limited stability of NRC's Sr+ clock is 3 × 10-15at 1 sec averaging time. The fractional evaluated accuracy of the Sr+ ion clock has reached 1.2 × 10-17and is now dominated by the BBR shift uncertainty, the improved evaluation of which is currently in progress.
Stability is one of most important performances of an atomic clock. Here we describe our recent work on improving the stability of our 40 Ca + optical clock. State preparation is adopted to transfer the ion to the ground-state magnetic sublevel of the clock transition, after the quenching laser transfers the ion to the ground state at each cycle. Using this method, the stability for 40 Ca + optical clock is improved to about . Compared with in previous work, the averaging time is decreased to reach a given level of statistical uncertainty in a clock comparison .
Optical atomic clocks have been rapidly developing in recent decades, resulting in major improvements in both precision and accuracy. As a result, they have become instrumental in multiple areas of applied and fundamental research. Despite all atomic frequency references having more than two energy-levels, the commonly used model for evaluating their ultimate limits assumes a two-level atom. This leads to frequency interrogation protocols and theoretical stability bounds that are suboptimal for a true multi-level atom. The most fundamental stability bound assumes two noise sources - quantum projection noise and spontaneous decay from the excited state. In this work, we analyze a model that includes these noise types and is generalized beyond the two-level assumption, where spontaneous decay can branch to more than a single ground state. This model allows for detection and exclusion of atomic frequency interrogations in which the atom decayed, leading to a frequency stability improvement of up to ≈4.5 dB\approx 4.5 \text{ dB}≈4.5 dB compared with the two-level model. Furthermore, we identify an even greater stability enhancement of ≈5.4 dB\approx 5.4 \text{ dB}≈5.4 dB for frequency comparisons between atoms in an odd parity Bell state. These enhancements are particularly relevant for the numerous trapped-ion optical clock species that operate close to lifetime-limited stability. We calculate new stability limits for those cases and provide a detailed experimental protocol for frequency interrogation with an 27Al+^{27}\text{Al}^{+}27Al+ optical ion clock.
The frequency of the 5s 2S(1/2)-4d 2D(5/2) electric quadrupole clock transition in a single, trapped, laser-cooled 88Sr+ ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cesium standard.
We describe the recent advances of the NRC single ion based optical frequency standard using the 445-THz S-D transition in88Sr+. Laser cooled confinement and probing of the clock transition has been obtained for periods exceeding 24 hours showing high levels of stability. Spectral resolutions better than 5 Hz at 445 THz have been obtained. Work is described on the development of an improved ion trap system and the continuous linkage of the optical frequency to the RF domain using a fiber based frequency comb.