High-performance atomic timekeeping with integrated iodine clocks and the GNS satellite system, VIPER, EPIC and PICLOSE

Molecular iodine (I2) has a legacy as an optical frequency standard13,14,15,16,17. One of the first demonstrations with opticalclock19,20 included several iodine transitions which were officially recognized as length standards. There have been studies on the issue of iodine frequencies for space missions. Here we report the deployment of several high-performance, fully integrated iodine optical clocks and highlight their ability to maintain nanosecond (ns)-level timing for several days while continuously operating at sea.

Atomic timekeeping is an essential part of modern infrastructure. Billions of devices rely on the Global navigation satellite system. The Global Navigation Satellite System is a network of distributed, high-performance microwave-based atomic clocks that provide nanosecond-level synchronization globally. The emergence of fieldable optical timekeeping, which offers short timescales and multi day holdover, along with long-distance optical time transfer12, paves the way for global synchronized at picosecond levels.

VIPER exhibits a short-term instability of 1.3 × 10−13/(\sqrt{\tau }) as well as a more prominent diurnal temperature instability that peaks at 4 × 10−14 near 40,000 s (corresponding to roughly 1 day periodic instability). The temperature coefficients of the three clocks are different and this is because of the relaxation of performance goals in theviper physics package. Nonetheless, this system can average over the diurnal temperature fluctuation and maintain an instability of 2.5 × 10−14 after 1 day of averaging. VIPER showed a drift rate similar to PICKLES and EPIC during the underway. Importantly, the VIPER physics package does not include magnetic shields yet still provides excellent frequency stability despite motion through Earth’s magnetic field.

A broad feature with a peak deviation of 4 × 10−15 is evident in the PICKLES Allan deviation at roughly 20,000 s (about 7 h) timescales. The equivalent optical frequency deviation of 2 Hz corresponds to a shift of about 2 ppm of the hyperfine transition line centre. We think that this is the origin of PICKLES, because the ram is coming through the etalon. By modifying the build procedure, this etalon was mitigated during the build of the EPIC spectrometer.

The iodine clock exhibits excellent phase noise for the 10 and 100 MHz tones derived by optical frequency division as well as the 1,064 nm optical output (Fig. 1b). The phase noise at microwave frequencies is lower than commercial atomic-disciplined oscillators, highlighting the benefits of optical frequency division where the fractional noise of the iodine-stabilized laser is transferred to the frequency comb repetition rate.

A portable atomic clock based on iodine molecules to distinguish between Earth’s magnetic and geomagnetic fields at 270 mG

The vessel travelled in all four cardinal directions during the exercise, illustrated by the GPS-tracked trajectory in Fig. 3c. The National Oceanic and Atmospheric Administration geomagnetic model for Earth’s magnetic field at this latitude and longitude shows that the projection of the Earth’s field on the clocks varied by ±270 mG throughout the underway (https://www.ngdc.noaa.gov/geomag/geomag.shtml).

There is potential for environmental sensitivities to be tied to ship dynamics, motion in Earth’s magnetic field, and temperature and humidity inside the Conex. Standard reference clocks (such as a caesium beam clock or GPS-disciplined rubidium) were not available for comparison. The level of common mode rejection required to mask fluctuations is raised by the simultaneous evaluation of three clocks. Pairing the at-sea test data of three clocks with environmental testing on land provides confidence that potential correlations are below the measured instability (Supplementary Information).

The clock’s tick is based on radiation that atoms emit and absorb as they oscillate between energy states. The clock is made from atoms of caesium and other elements that emit radiation. Some are portable and are sold commercially.

Donley says this stability is similar to that of a hydrogen maser clock — a reliable kind of microwave atomic clock that is the workhorse for international timekeeping. The clock is strong and around a tenth of the volume.

The clock’s robustness comes in part from its use of iodine molecules, which can be made to oscillate using compact and durable lasers of the type commonly used in labs. According to the physicist Martin Boyd, themolecules are less sensitive than some atoms to pressure and temperature fluctuations.

Future models could fly on global navigation satellites and improve position resolution from metres to centimetres if the team can shrink the clock further. He says they could be the clock that defines lunar time.

Such a precise but portable clock could be used to improve research that requires precise timing in the field, including mapping Earth’s gravitational field and using multiple telescopes to image black holes.

The clock, which was detailed in a paper in Nature on 24 April1, could also provide a “vital fallback solution” if signals from global navigation systems are spoofed or jammed in conflict zones, says Tetsuya Ido, director of the Space-Time Standards Laboratory at the Radio Research Institute in Tokyo.

Elizabeth Donley is the head of the time and frequency division at the US National Institute of Standards and Technology. We are excited to get our hands on it.