Sr lattice clock at 1x10^{-16} fractional uncertainty by remote optical evaluation with a Ca clock (0801.4344v2)

Abstract: Optical atomic clocks promise timekeeping at the highest precision and accuracy, owing to their high operating frequencies. Rigorous evaluations of these clocks require direct comparisons between them. We have realized a high-performance remote comparison of optical clocks over km-scale urban distances, a key step for development, dissemination, and application of these optical standards. Through this remote comparison and a proper design of lattice-confined neutral atoms for clock operation, we evaluate the uncertainty of a strontium (Sr) optical lattice clock at the 1x10-16 fractional level, surpassing the best current evaluations of cesium (Cs) primary standards. We also report on the observation of density-dependent effects in the spin-polarized fermionic sample and discuss the current limiting effect of blackbody radiation-induced frequency shifts.

• The paper demonstrates a Sr optical lattice clock achieving 1x10⁻¹⁶ fractional uncertainty through remote comparison with a Ca clock.
• It employs phase-coherent 4-km fiber links and stabilized optical frequency combs to minimize systematic uncertainties effectively.
• These advancements pave the way for enhanced timekeeping, fundamental physics tests, and future integration of quantum optics techniques.

Evaluation of Strontium Optical Lattice Clocks at 1x10⁻¹⁶ Fractional Uncertainty

The paper by Ludlow et al. presents a significant advancement in the field of optical atomic clocks, specifically focusing on strontium (Sr) optical lattice clocks. Traditional timekeeping has been heavily reliant on cesium (Cs) atomic clocks; however, with the continued progress in optical frequency standards, the landscape is shifting. This paper is indicative of this shift, demonstrating a neutral Sr atomic standard with a fractional uncertainty measurement of 10⁻¹⁶.

Methodology and Experimental Setup

The paper details the systematic uncertainty evaluation for a Sr optical lattice clock, surpassing the best traditional Cs primary standards. This optical standard utilizes a large ensemble of strontium atoms confined in a one-dimensional optical lattice. The authors discuss their approach to phase-coherently transfer the Sr clock's timing information across a 4-km fiber optic link to a calcium (Ca) optical clock at the National Institute of Standards and Technology (NIST). This remote comparison mitigates the inherent complexity and availability issues of deploying multiple optical clocks within a single space.

A key feature of the setup involves the use of a coherent optical carrier transfer system, stabilized with optical frequency combs, enabling high precision measurements without significant deterioration from the optical Dick effect, despite dead times during measurements. The ensemble approach, facilitated by laser cooling to 2.5 µK and spin polarization to pure states, exhibits a high-degree control over state preparation and interrogation, essential for minimizing uncertainties.

Key Results and Precision Observed

The paper achieves remarkable precision in atomic confinement and clock operation, demonstrating stability figures approaching the fundamental quantum noise limit. The Sr clock is analyzed against a Ca clock with superior resonance quality factors, reaching stability levels below 3x10⁻¹⁶ after 200 seconds. The optical confinement occurs at a "magic" wavelength, where polarizabilities are symmetrical, effectively canceling otherwise prevalent vector light shifts.

Comprehensive evaluation of systematic uncertainties revealed a variety of contributing factors including lattice Stark effects, blackbody radiation (BBR) shifts, ac Stark shifts, Zeeman shifts, and atomic density-related shift. Notably, the total systematic uncertainty is refined to 1.5x10⁻¹⁶, representing considerable progress over previous measurements.

Implications and Future Directions

This work advances the capability of employing optical standards for not only improving international timekeeping but also enabling precise investigations of fundamental physics, such as tests of fundamental physical constants and relativistic frequency shifts over large distances.

Moreover, the development of large ensemble techniques with rigorous control over state preparation in lattice-confined systems suggests potential for integrating quantum optics strategies, such as spin squeezing, into high-precision measurements. Future research could explore systematic enhancements, particularly in reducing BBR-related uncertainties, through the deployment of well-characterized blackbody environments.

This paper thus underscores the profound implications of optical atomic standards, suggesting a near-future transition where they may headline advancements in metrology and fundamental physics. The technical achievement of such remote comparisons highlights the necessary step toward deploying optical clocks for applications such as geodetic measurements and global positioning advancements, thus paving the way for future innovation in timekeeping.