Abstract
Optical atomic clocks have demonstrated revolutionary advances in precision timekeeping, but their applicability to the real world is critically dependent on whether such clocks can operate outside the laboratory. Photonic integration offers one compelling solution to address the miniaturization and ruggedization needed to enable clock portability, but brings with it a new set of challenges in recreating the functionality of an optical clock using chip-scale building blocks. The clock laser used for atom interrogation is one particular point of uncertainty, as the performance of the meticulously engineered bulk-cavity-stabilized lasers would be exceptionally difficult to transfer to chip. Here we demonstrate that an integrated ultrahigh-quality-factor spiral cavity, when interfaced with a 1,348 nm seed laser, is able to reach a fractional frequency instability of 7.5 × 10−14 on chip. On frequency doubling the light to 674 nm, we use this laser to interrogate the narrow-linewidth transition of 88Sr+ and showcase the operation of a Sr-ion clock with short-term instability averaging down as \(3.9\times 1{0}^{-14}/\sqrt{\tau }\) (τ, averaging time). Our demonstration of a high-performance optical atomic clock interrogated by an integrated spiral cavity laser opens the door for future advanced clock systems to be entirely constructed using lightweight, portable and mass-manufacturable integrated optics and electronics.
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Data availability
The datasets that support this study are available from the corresponding authors on reasonable request. Source data are provided with this paper.
Code availability
The code used for analysis and simulations are available from the corresponding authors on reasonable request.
Change history
17 February 2025
A Correction to this paper has been published: https://doi.org/10.1038/s41566-025-01643-y
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Acknowledgements
This work was sponsored by the Under Secretary of Defense for Research and Engineering under Air Force contract no. FA8721-05-C-0002 and by the Defense Advanced Research Projects Agency (DARPA) under contract number FA8702-15-D-0001. The views, opinions and/or findings expressed are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the US Government. Distribution Statement “A” (Approved for Public Release, Distribution Unlimited).
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W.L., D.R., R.M. and D.G. conceived, designed and carried out the experiments with the ISCL. D.R., R.M., W.L. and W.S. conceived, designed and carried out the experiments with the clock protocol. D.K. and A.S. fabricated the spiral resonators. C.B., R.T.M. and A.M. prepared the fibre-attached spiral resonator. C.D.B. and D.R. developed the fibre and vibration noise cancellation for the clock system. All authors discussed the results and contributed to the paper.
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Extended data
Extended Data Fig. 1 Spiral Resonator Component Sections.
Plot of the spiral resonator with its different component sections highlighted. The number of revolutions is reduced from 25 to 2 for clarity. The spiral resonator comprises the following sections: pure circles, central cubics, inward going racetrack, outward going racetrack, a rotated clone of all the preceding, racetrack interconnects, and bus waveguide.
Extended Data Fig. 2 Integrated Spiral Cavity Laser Optical System.
Simplified system diagram showcasing the optical components of the ISCL. Photographs of the corresponding optical devices in the setup are also displayed.
Extended Data Fig. 3 Optimization of ISCL Operation.
a, Frequency noise spectra measured at a variety of chip-coupled optical powers. The line noise at 180 Hz and 300 Hz reduces with optical power. However, at 5.1 mW, the broadband noise level increases sharply. b, Noise at the power line frequencies of 60 Hz, 180 Hz, and 300 Hz plotted versus the on-chip optical power. The measured power line noise at all three frequencies decreases with increasing power. c, Fractional frequency noise of the ISCL across the same range of optical powers previously tested. Changes in optical power lead to competing effects in the stability of the laser at both short and longer averaging times. d, Fractional frequency noise versus optical power for averaging times of 1 ms and 30 ms. The optimum chip-coupled power is near 4.1 mW.
Extended Data Fig. 4 ISCL Fiber Link Noise.
a, Phase noise of ISCL measured at the output of the ISCL at 1348 nm and across the fiber link to the Sr-ion laboratory after frequency doubling to 674 nm (scaled to 1348 nm for comparison). The noise increases slightly around 10 Hz-1 kHz offset frequencies. b, Fractional frequency instability measured for the 1348 nm ISCL before and after the fiber link. The fiber noise is most prominent near 10 ms averaging time.
Extended Data Fig. 5 Experimental Tests of Atom Locking With Integration.
Tests of the atom lock with τint = 1 ms and an intentionally applied 0.1 Hz sinusoidal modulation having an amplitude that increases over time. The trial where integration is included in the locking protocol (α = 0.13, β = 0.0035, and γ = 0.995) outperforms the case with no integration (α = 0.2).
Extended Data Fig. 6 Atom Measurements Using the ISCL.
a, Narrow spectroscopy using Rabi pulses was performed with the atom and the ISCL. A 2.5 ms probe pulse is used, increasing the dead time of the clock and requiring greater performance. The observed linewidth of 380 Hz is Fourier limited by the interrogation time. The scan is performed with 400 trials per point and was obtained in ∼2 minutes. b, Rabi oscillation spectroscopy performed with the ISCL. The fit demonstrates π-pulse time of 11 μs, with the observed decoherence due to the thermal state of the ion corresponding to mean axial mode occupation \(\bar{n}\approx 19.5\).
Source data
Source Data Fig. 1
Spiral resonator curvature.
Source Data Fig. 2
ISCL noise.
Source Data Fig. 3
Spiral resonator curvature.
Source Data Fig. 4
Sr-ion clock measurements.
Source Data Extended Data Fig. 3
ISCL power comparison measurements.
Source Data Extended Data Fig. 4
Fibre link noise.
Source Data Extended Data Fig. 5
Integration tests for atom locking.
Source Data Extended Data Fig. 6
Atom measurements.
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Loh, W., Reens, D., Kharas, D. et al. Optical atomic clock interrogation using an integrated spiral cavity laser. Nat. Photon. 19, 277–283 (2025). https://doi.org/10.1038/s41566-024-01588-8
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DOI: https://doi.org/10.1038/s41566-024-01588-8
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