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Demonstration of a trapped-ion atomic clock in space

Abstract

Atomic clocks, which lock the frequency of an oscillator to the extremely stable quantized energy levels of atoms, are essential for navigation applications such as deep space exploration1 and global navigation satellite systems2, and are useful tools with which to address questions in fundamental physics3,4,5,6. Such satellite systems use precise measurement of signal propagation times determined by atomic clocks, together with propagation speed, to calculate position. Although space atomic clocks with low instability are an enabling technology for global navigation, they have not yet been applied to deep space navigation and have seen only limited application to space-based fundamental physics, owing to performance constraints imposed by the rigours of space operation7. Methods of electromagnetically trapping and cooling ions have revolutionized atomic clock performance8,9,10,11,12,13. Terrestrial trapped-ion clocks operating in the optical domain have achieved orders-of-magnitude improvements in performance over their predecessors and have become a key component in national metrology laboratory research programmes13, but transporting this new technology into space has remained challenging. Here we show the results from a trapped-ion atomic clock operating in space. On the ground, NASA’s Deep Space Atomic Clock demonstrated a short-term fractional frequency stability of 1.5 × 10−13/τ1/2 (where τ is the averaging time)14. Launched in 2019, the clock has operated for more than 12 months in space and demonstrated there a long-term stability of 3 × 10−15 at 23 days (no drift removal), and an estimated drift of 3.0(0.7) × 10−16 per day. Each of these exceeds current space clock performance by up to an order of magnitude15,16,17. The Deep Space Atomic Clock is particularly amenable to the space environment because of its low sensitivity to variations in radiation, temperature and magnetic fields. This level of space clock performance will enable one-way navigation in which signal delay times are measured in situ, making near-real-time navigation of deep space probes possible18.

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Fig. 1: The Deep Space Atomic Clock launch.
Fig. 2: Clock vacuum chamber and traps.
Fig. 3: Clock frequency offsets in space.
Fig. 4: Clock stability in space.

Data availability

Full data for Fig. 4 and Extended Data Fig. 3 are available from the corresponding author on reasonable request.

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Acknowledgements

This work is jointly funded by NASA’s Space Technology Mission Directorate (STMD) office and the office of Space Communications and Navigation (SCaN). The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Copyright 2020, California Institute of Technology. Government sponsorship acknowledged.

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Authors

Contributions

E.A.B., J.D.P. and R.L.T. contributed equally. E.A.B. led DSAC clock preparation and optimization, determined operation modes and parameters, characterized the clock and payload before launch and in space, and led manuscript preparation activity. J.D.P. is the DSAC clock co-investigator, led the vacuum tube, ion trap, optics and magnetic design, and consulted on all other aspects of clock preparation, characterization and data analysis. R.L.T. is the DSAC clock co-investigator, consulted on all aspects of clock and payload design, preparation, characterization and data analysis, and led efforts to resolve several technical challenges with the instrument. D.G.E. led the development of telemetry processing and analysis tools, characterized local oscillator and system performance, and performed USO and control loop modelling. D.K. contributed GPS data processing, modelling and analysis. D.W.M. led the GPS data processing, modelling and analysis, including relativistic calculations and orbital parameter analysis, and wrote telemetry processing and display software. D.E.R. led controller algorithm implementation and data acquisition, and coordinated clock commanding through the spacecraft provider. J.M.S. was the DSAC deputy principal investigator and led technical and programmatic planning and performed the new one-way navigation experiments enabled by DSAC. T.A.E. is the DSAC principal investigator, led technical and programmatic planning and performed the new one-way navigation experiments enabled by DSAC, and with D.W.M. devised new methods to perform GPS receiver thermal calibrations. R.T.W led USO characterizations, instrument integration into the payload and payload integration into the spacecraft. All authors read, edited and approved the final manuscript.

Corresponding author

Correspondence to E. A. Burt.

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The authors declare no competing interests.

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Peer review information Nature thanks Pascale Defraigne, Ekkehard Peik and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 System-level block diagram of the DSAC payload.

The GPS receiver (Rcv.) is not shown.

Extended Data Fig. 2 Level diagram for 199Hg(II).

The S1/2, F = 1 to P1/2, F = 1 optical electric dipole transition used for state preparation and readout is shown in purple (note that the source of this light is broad enough to not resolve Zeeman sublevels, so the line is meant to indicate any transition between the S1/2 F = 1 and P1/2 F = 1 manifolds that is consistent with selection rules). Also shown are the S1/2, F = 0, mF = 0 to S1/2, F = 1, mF = 0 magnetic-field-insensitive 40.5-GHz hyperfine clock transition (thick black arrow), and the ΔmF = ±1 field-sensitive Zeeman lines at ±140 kHz (thin black arrows).

Extended Data Fig. 3 Measured Allan deviation.

Allan deviation without any corrections (red), with relativity corrections but no temperature corrections (green), and with both relativity and temperature corrections (black). Simulated expected clock performance with clock parameters during the run with environmental perturbations (solid blue) and without (dashed blue) is shown for comparison. All traces except the blue are overlapping Allan deviation. For reference, the orbital period of about T = 6,000 s will result in expected peaks in the Allan deviation at 0.37T ≈ 2,200 s (ref. 30).

Extended Data Fig. 4 Variations in the Earth’s magnetic field observed on the DSAC spacecraft as a function of time.

The orbital period is approximately 6,000 s at an altitude of 720 km.  The component of the fieldin the weakest shielding direction is plotted.  Shielding in the other two directions is over an order of magnitude higher so that the impact of variations on the clock is dominated by the component shown.

Extended Data Fig. 5 Impact of magnetic field variations on the clock.

The Allan deviation of the frequency shift associated with measured magnetic field variations on board the DSAC spacecraft is shown (red) assuming a worst-case sensitivity of 7 × 10−16 μT−1. Reference lines are also shown for expected multipole trap (dashed blue) and load trap (dashed black) operation noise floors (without LO noise aliasing effects for the multipole trap line). Error bars represent 68% confidence intervals.

Extended Data Fig. 6 Fractional frequency versus signal size in the load trap.

Signal size is the number of PMT counts measured at the line centre minus the counts at a detuning of one linewidth corresponding to the first minimum in a Rabi line trace. The slope gives an estimate of the number-dependent second-order Doppler shift while operating in the load trap. As a point of reference, the corresponding shift in the multipole trap would be 10–20 times smaller.

Extended Data Fig. 7 Residuals from a fit to frequency versus signal size.

Residuals are now plotted against temperature in the load trap. A linear fit gives a total temperature sensitivity of −2.3(1.1) × 10−15 °C−1.

Extended Data Fig. 8 Temperature data.

Temperature data shown for the 52-day dataset described in the main text. a, Long-term temperature variation in the load trap over 52 days correlated with changes in Sun beta angle. b, A 5-day subset showing 24-h temperature variations. c, A 1-day subset showing orbital temperature variation.

Extended Data Fig. 9 Trace-gas evolution.

Frequency data with the temperature effect removed (black dots) and fitted to a straight line (blue line), so as to place a limit of 4.6 × 10−16 per day on frequency shifts due to trace-gas evolution in the clock vacuum chamber.

Extended Data Fig. 10 Passage through the SAA.

Total PMT counts in 8.1-s portions of each clock cycle as a function of time, showing excess counts due to passage through the SAA. Each passage is approximately 20 min, as shown for the expanded view (inset). The varying peak amplitude is due to spacecraft orbital precession, which varies the trajectory of the spacecraft in and out of the SAA on a daily timescale.

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Burt, E.A., Prestage, J.D., Tjoelker, R.L. et al. Demonstration of a trapped-ion atomic clock in space. Nature 595, 43–47 (2021). https://doi.org/10.1038/s41586-021-03571-7

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