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Two-photon rubidium clock detecting 776~nm fluorescence

River Beard, Kyle W. Martin, John D. Elgin, Brian L. Kasch, Sean P. Krzyzewski

TL;DR

The study demonstrates the first two-photon rubidium clock stabilized by detecting 776 nm fluorescence, achieving a 2.6× improvement in short-term instability over a 420 nm detection scheme and validating a low-voltage MPPC as a practical alternative to PMTs in the feedback loop. By leveraging 776 nm fluorescence from the $5D_{5/2} \rightarrow 5P_{3/2}$ cascade, the work reduces radiation trapping and enhances signal, while preserving a robust, compact architecture with a telecom-derived clock laser and an optical comb reference. Environmental sensors are integrated to bound systematic shifts from collisional, ac-Stark, and Zeeman perturbations, yielding a comprehensive error budget that supports the approach as a viable path toward portable optical frequency standards. The results, together with the MPPC implementation, highlight practical routes to SWaP-compatible, high-stability clocks capable of expanding detection wavelengths beyond traditional PMT-based schemes.

Abstract

The optical atomic clock based on the $5S_{1/2} \rightarrow 5D_{5/2}$ two-photon transition in rubidium is a candidate for a next generation, manufacturable, portable clock that fits in a small size, weight, and power (SWaP) envelope. Here, we report the first two-photon rubidium clock stabilized by detecting 776~nm fluorescence. We also demonstrate the use of a multi-pixel photon counter as a low voltage substitute to a photomultiplier tube in the feedback loop to the clock laser.

Two-photon rubidium clock detecting 776~nm fluorescence

TL;DR

The study demonstrates the first two-photon rubidium clock stabilized by detecting 776 nm fluorescence, achieving a 2.6× improvement in short-term instability over a 420 nm detection scheme and validating a low-voltage MPPC as a practical alternative to PMTs in the feedback loop. By leveraging 776 nm fluorescence from the cascade, the work reduces radiation trapping and enhances signal, while preserving a robust, compact architecture with a telecom-derived clock laser and an optical comb reference. Environmental sensors are integrated to bound systematic shifts from collisional, ac-Stark, and Zeeman perturbations, yielding a comprehensive error budget that supports the approach as a viable path toward portable optical frequency standards. The results, together with the MPPC implementation, highlight practical routes to SWaP-compatible, high-stability clocks capable of expanding detection wavelengths beyond traditional PMT-based schemes.

Abstract

The optical atomic clock based on the two-photon transition in rubidium is a candidate for a next generation, manufacturable, portable clock that fits in a small size, weight, and power (SWaP) envelope. Here, we report the first two-photon rubidium clock stabilized by detecting 776~nm fluorescence. We also demonstrate the use of a multi-pixel photon counter as a low voltage substitute to a photomultiplier tube in the feedback loop to the clock laser.

Paper Structure

This paper contains 12 sections, 4 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Fluorescence at 776 nm
  3. Experiment
  4. Laser System
  5. Rubidium Atomic Reference
  6. Multi-Pixel Photon Counter
  7. Results
  8. Collisional Shift
  9. ac-Stark Shift
  10. Zeeman Shift
  11. Other Effects
  12. Discussion

Figures (4)

  • Figure 1: Relevant energy levels of rubidium and branching ratios cascaded from the 5D$_{5/2}$ state using individual decay branching ratios from UDportal.
  • Figure 2:
  • Figure 3: Time and Allan deviation of two-photon rubidium clock while detecting 776 nm fluorescence and while detecting 420 nm fluorescence. Frequency instabilities are plotted with solid lines. Integrated timing errors are plotted with dashed lines.
  • Figure 4: Overlapping Allan deviations of cell temperature and optical power witness data, rescaled to the fractional frequency stability limit they individually impose on the clock based on conversion factors taken from Martin2018Lemke2022. Instabilities from the ac-Stark shift are plotted with circles, instabilities from collisional shifts are plotted with squares, and instabilities from the sum of the shifts are plotted with triangles.