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Chinese Physics Letters

Improvement of Stability of 40 Ca + Optical Clock with State Preparation

Mengyan Zeng, Yao Huang, H. Shao, Miao Wang, Huaqing Zhang, Bao-Lin Zhang, Hua Guan, Kelin Gao

Published Jul 1, 2018• 6 citations
Publisher

Abstract

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 .

DOI
10.1088/0256-307x/35/7/074202

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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 .
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.
Abstract Here, we develop a compact multilayer magnetic shield for a transportable optical clock, achieving shielding factors of up to 10 5 . To improve the frequency stability of the transportable optical clock, we improved the magnetic-field environment of a transportable 40 Ca + ion optical clock (TCIOC) using the newly developed magnetic shielding system and reduced the clock-laser frequency noise by transferring the stability of an ultrastable reference laser through an optical frequency comb. These improvements enabled the coherent interrogation time to be extended from 40 ms to 320 ms . A time-interleaved self-comparison yielded a measured short-term fractional frequency instability of 1.6 × 10 − 15 / τ , close to the estimated quantum projection noise-limited instability of 1.18 × 10 − 15 / τ . Meanwhile, the uncertainty contributions associated with the DC second-order Zeeman shift and the servo error were reduced to 1.1 × 10 − 21 and 4.5 × 10 − 19 . These results provide a practical basis for further improving the performance of transportable optical clocks.
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.
The fractional uncertainty of the 40Ca+ optical clock in our laboratory has been evaluated to be 2.2×10−17, but it is difficult to evaluate the statistical uncertainty of the output frequency at this level, limited by the frequency stability associated with the quantum projection noise (QPN). A reduction in the stability is required to shorten the averaging time for a certain statistical uncertainty level. To improve the stability, the Ramsey method is introduced in the locking process. To improve the reliability and continuity, we introduce the idea of using auxiliary Rabi probes to avoid fringe slips in the Ramsey interrogation and to automatically search for resonance. The stability of a single 40Ca+ ion frequency standard is improved from 6.3×10−15/τ to 2.5×10−15/τ, where τ is the averaging time in seconds, which is close to the QPN limit for an 80-ms-long Ramsey interrogation. We obtain a stability of 1.5×10−17 at an averaging time of 21 000 s and an uptime rate of 93.8% for a 34-h-long run. These methods can easily be applied to other ion or neutral atom optical clocks.
Abstract Optical clocks are important for precise measurements in the field of physics. As reported, both the instability and uncertainty of optical lattice clocks are more than two orders of magnitude smaller than those of the best microwave clocks. Therefore, in the near future, optical clocks could be used to redefine the second. Nevertheless, an optical clock with reliability comparable to microwave clocks has not been achieved thus far. In this paper, we compared the frequencies of two Ca^+ optical clocks that were nearly continuously operated for 31 days. Through the comparison experiment, the frequency stability of a single clocks was found to be 6.3×10^-18 at an averaging time of 520 000 s and 7.9×10^-18 at averaging time of 262000 s, while the operating uptime rate reached more than 90% in the period of around 5 days. Thus, our experiment demonstrated that it is possible to increase the stability of single-ion optical clocks to the 10^-18 level, while still maintaining quasi-continuous operation with a high operating rate. This result further confirms that optical clocks can potentially be used to redefine the second.
Entangled states hold the promise of improving the precision and accuracy of quantum sensors. We experimentally demonstrate that spectroscopy of an optical clock transition using entangled states can outperform its classical counterpart. Two ^{40}Ca^{+} ions are entangled in a quantum state with vanishing first-order magnetic field sensitivity, extending the coherence time of the atoms and enabling near-lifetime-limited probe times of up to 550 ms. In our protocol, entangled ions reach the same instability as uncorrelated ions, but at half the probe time, enabling faster cycle times of the clock. We run two entangled ^{40}Ca^{+} ions as an optical clock and compare its frequency instability with a ^{87}Sr lattice clock. The instability of the entangled ion clock is below a clock operated with classically correlated states for all probe times. We observe instabilities below the theoretically expected quantum projection noise limit of two uncorrelated ions for interrogation times below 100 ms. The lowest fractional frequency instability of 7×10^{-16}/sqrt[τ/1 s] is reached for 250 ms probe time, limited by residual phase noise of the probe laser. This represents the lowest instability reported to date for a ^{40}Ca^{+} ion clock.