Session P2a: Advanced and Future Clocks

Rb Microcell Atomic Clock Based on a Micro-Loop-Gap Microwave Resonator
Christoph Affolderbach, Laboratoire Temps-Fréquence, University of Neuchâtel; Yuanyan Su, Microwave and Antenna Group, Ecole Polytechnique Fédérale de Lausanne (EPFL); Matthieu Pellaton, William Moreno, Laboratoire Temps-Fréquence, University of Neuchâtel; Anja K. Skrivervik, Microwave and Antenna Group, EPFL; Gaetano Mileti, Laboratoire Temps-Fréquence, University of Neuchâtel
Location: Seaview A/B

Objectives
Miniature or Chip-Scale Atomic Clocks (CSAC) have become well-established references for applications such as underwater prospection or holdover applications, thanks to their typically 1E-11 level frequency stabilities over several hours’ timescale (1 microsecond/day), small volume and low power consumption. There is however an emerging need for improved miniature atomic clocks with improved short-term stabilities of 1E-10 to 1E-11 /sqrt(tau) (tau=1 to 100 s) and a long-term frequency drift of <1E-11 to 1E-12 /day, for instance for positioning, navigation and timing (PNT) applications or secure communications. In order to achieve this improvement in stability over existing CSAC by approximately one order of magnitude, we propose to exploit the optical-microwave Double-Resonance (DR) scheme that assures both a high signal contrast (thus superior short-term clock stability) as well as low light-shift effects, which can compromise long-term clock stability in alternative schemes such as CPT that are generally employed in existing CSAC.
Clock design and realization
In order to achieve an overall low volume and power consumption of the miniature DR clock, our clock approach, named the LEMAC miniature atomic clock, is based on only three key components: a micro-fabricated Rb cell that provides the atomic reference sample, a custom-designed micro-loop-gap microwave resonator (µ-LGR) to apply microwave radiation to the atomic reference transition, and a low-power laser source providing the interrogation light. The basic design of the LEMAC clock was presented at the joint EFTF/IFCS-2022 conference.
A key development for the LEMAC clock is the micro-LGR. This miniature microwave resonator is based on a stack of micro-fabricated planar PCBs on which thin copper electrodes (loops) interrupted by small gaps are patterned. These loop-gap electrodes form an effective LC-resonance circuit with a resonance at the 6.8 GHz Rb ground-state frequency, but from a structure of sub-cm size (far below the radiation wavelength) as required for the envisaged miniaturization. This micro-LGR has a total volume of < 0.65 cm^3 and surrounds the Rb microcell held in its center, across which it guarantees a highly uniform microwave field distribution to achieve a high-quality clock signal. The tuning-free design of the micro-LGR assures operation at resonance without need for frequency adjustment, even in view of manufacturing tolerances, for high production yield.
For the LEMAC clock operation, a low-power laser source provides light at the 795nm Rb D1-line that is sent through the Rb microcell for optical pumping of the Rb ground state. A part of the transmission signal through the microcell is used for frequency-stabilization of the laser light to the optical atomic transition. When the microwave radiation applied via the micro-LGR is resonant with the Rb reference transition, this is observed as a change in light intensity transmitted through the cell and this signal is used to steer the frequency of the clock’s quartz oscillator. The fully assembled LEMAC clock demonstrator (physics package) has an overall volume of 86 x 86 x 48 mm (< 360 cm^3 volume) of which less than half the volume is occupied by the clock physics, the rest being due to electronics’ connectors that connect to laboratory-style clock electronics at this stage.
Results
The clock characterization and measurements were performed under standard laboratory conditions under ambient atmosphere (clock not placed in vacuum). The current clock signal has a linewidth of 11 kHz and a contrast (amplitude over background) around 5%. This results in a measured short-term clock stability of approximately 2E-11 /sqrt(tau) at 1 to 100 s, in good agreement with the estimated signal-to-noise limit. The long-term clock stability is measured as around 4E-12 at 1 day, with a low frequency drift of 2E-13 /day.
At the conference we will discuss the main stability limitations for the LEMAC clock, including instability budgets at both short-term and long-term timescales. Prospects for future improvements will be discussed.
This activity was conducted and was funded under the Navigation Innovation and Support Program (NAVISP) of the European Space Agency (ESA), to support innovation and competitiveness in the field of Position, Navigation and Time (ESTEC contract nr 4000129974).