Progress in Atomic Fountains at LNE-SYRTE

Abstract: We give an overview of the work done with the Laboratoire National de M\'etrologie et d'Essais-Syst`emes de R\'ef\'erence Temps-Espace (LNE-SYRTE) fountain ensemble during the last five years. After a description of the clock ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest developments, we review recent studies of several systematic frequency shifts. This includes the distributed cavity phase shift, which we evaluate for the FO1 and FOM fountains, applying the techniques of our recent work on FO2. We also report calculations of the microwave lensing frequency shift for the three fountains, review the status of the blackbody radiation shift, and summarize recent experimental work to control microwave leakage and spurious phase perturbations. We give current accuracy budgets. We also describe several applications in time and frequency metrology: fountain comparisons, calibrations of the international atomic time, secondary representation of the SI second based on the 87Rb hyperfine frequency, absolute measurements of optical frequencies, tests of the T2L2 satellite laser link, and review fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally, we give a summary of the tests of the PHARAO cold atom space clock performed using the FOM transportable fountain.

• The paper presents systematic advancements in the FO1, FO2, and FOM clocks by refining methods to reduce distributed cavity phase shifts and achieve uncertainties near 10⁻¹⁷.
• It employs innovative methodologies, including dual-species operation and transportable designs, to enhance clock accuracy and broaden their practical applications.
• The findings significantly improve calibrations for international atomic time and support fundamental physics research by pushing the limits of precision time measurement.

Overview of Atomic Fountain Advancements at LNE-SYRTE

The paper "Progress in Atomic Fountains at LNE-SYRTE" by Guéna et al. provides a detailed account of the developments and applications of atomic fountain clocks at the LNE-SYRTE institute, including systematic studies and metrological contributions. This paper serves as a comprehensive review of the achievements in building and refining three primary fountain clocks: FO1, FO2, and FOM. These devices are pivotal in improving the accuracy of time standards and testing fundamental physics.

Key Developments and Methodologies

The LNE-SYRTE team operates three distinct primary fountain clocks: FO1, FO2, and FOM, each serving specialized purposes and undergoing continuous improvements to refine their precision and reduce systematic uncertainties. The primary developments are as follows:

• FO1 Clock: As a $^{133}$Cs fountain established in 1994, FO1 incorporates a two-dimensional magneto-optical trap for enhanced atom loading. It exhibits no measurable tilt sensitivity in its cavity design and achieves a correction for the distributed cavity phase (DCP) shift standardized at $-9.7 \times 10^{-17}$ with total uncertainty.
• FO2 Clock: This dual fountain clock operates with $^{87}$Rb and $^{133}$Cs. Simultaneous operation ensures the independent processing of both atomic species without mutual interference. It reports an advanced DCP shift uncertainty of $-9.2 \pm 8.9 \times 10^{-17}$.
• FOM Clock: As a transportable fountain, FOM demonstrates flexibility and accuracy in external environments. It is crucial for experiments outside the laboratory setting and has shown remarkable results in calibrating international atomic time standards.

Systematic Shift Evaluations

The research explores evaluating several systematic frequency shifts, including:

• Distributed Cavity Phase Shift: This DCP shift in frequency possibly caused by phase variations in the microwave cavity is now better understood thanks to a combination of theoretical and experimental work. The newly proposed cavity designs aim to mitigate these shifts further.
• Microwave Lensing Effect: The study assesses how microwave standing waves modify atomic motions, causing minor frequency shifts. These assessments are becoming increasingly accurate, directly impacting the precision of timekeeping.
• Blackbody Radiation Shift: Measurement of the blackbody radiation effect on atomic hyperfine transitions remains a significant factor in clock accuracy. Improved measurements of the Stark coefficient allow LNE-SYRTE scientist to enhance predictions and corrections for these shifts, lending accuracy to the calibration of atomic clocks.

Implications in Metrology and Fundamental Physics

LNE-SYRTE's clocks offer crucial contributions to calibrating international atomic time (TAI), lending significant reliability and frequency calibration standards worldwide. Their research in the fields of gravity, quantum mechanics, and fundamental constants provides a testing ground for theory beyond existing capabilities.

Future Prospects

The paper outlines future developments aimed at reducing inaccuracies through advanced microwave cavity designs and further improving the reliability of the fountain ensemble. Another notable aspect is the potential in quantum-enhanced measurement techniques, which may push the boundaries of contemporary atomic clock stability.

In conclusion, LNE-SYRTE’s advancements in atomic fountain technology epitomize the cutting edge of time and frequency metrology, offering unprecedented accuracy and reliability. Their focus on understanding systematic shifts and new methodologies paves the way for future enhancements and breakthroughs in precision time measurement and fundamental physics investigations.