Skip to Main Content
Skip Nav Destination
Keeping accurate time while on the ocean

Keeping accurate time while on the ocean

8 May 2024

Researchers use iodine to design smaller optical clocks for uses outside the laboratory.

Three boxes with wires feeding out are shown stacked on top of each other.
Three optical atomic clocks, designated VIPER, EPIC, and PICKLES, were put out to sea for 20 days to test their stability. All remained stable and comparable to clocks used to determine universal coordinated time, despite not being in precisely controlled and stable laboratory conditions. Credit: Courtesy of Will Lunden

Maritime navigation for centuries has been intrinsically linked with the accuracy of clocks. The 18th century saw many design competitions for seaworthy clocks that retained their accuracy enough to determine a ship’s longitude. Nowadays, an accurate clock is important for participating in the Global Navigation Satellite System. Scientific and military vessels alike are equipped with microwave-based atomic clocks to achieve the precision necessary for safe sea travel. Such clocks, however, require a lot of external equipment to maintain their precision.

Optical atomic clocks, which provide higher performance than their microwave counterparts, have been primarily used in laboratory settings because they are bulky and sensitive to their surroundings. Researchers at Vector Atomic have now made a smaller optical clock that retains its precision outside the lab. The team, led by Martin Boyd, selected molecular iodine for the clock because its transitions are accessible with robust, industrial lasers. Several of its transition frequencies are also officially recognized as a length standard. Although using a molecule rather than an atom or an ion limits the accuracy of their clock, the researchers were aiming for a robust and mobile clock that is accurate enough for navigation.

A map of the Hawaiian Islands overlaid with the path taken by a ship.
A team from Vector Atomic tested three optical atomic clocks on a naval ship that sailed around the Hawaiian Islands in 2022 as part of the Rim of the Pacific, the world’s largest international maritime exercise. Credit: J. D. Roslund et al., Nature 628, 736, 2024/Vector Atomic

The Vector Atomic clock differs from typical optical atomic clocks in that it uses a vapor cell: an iodine-filled tube with glass windows that allows for laser excitation and detection of the oscillations within. Because vapor-cell architecture is generally immune to vibration and orientation and, unlike many atomic clock designs, does not use consumables, it is an excellent choice for precision measurement in the field. It also doesn’t require vacuum systems or cooling, thus further minimizing the accessory technologies needed to maintain the clock. Previous vapor-cell atomic clocks were only stable on a short time scale. The researchers counteracted that in part with a custom-built laser tailored for their clock.

The team made three clocks to test. Two identical ones were tested against high-precision clocks at NIST and were found to be more stable over measurement intervals of under a thousand seconds than an active hydrogen maser. The frequency drift of the Vector Atomic clocks was only slightly greater over longer time scales than a calibration clock. The third clock had less stringent technical requirements but was designed for reduced size and power requirements. The researchers were satisfied with the lab tests, and the clocks were put out to sea.

For 20 days the three clocks were sailed around the Hawaiian Islands during a naval exercise. Despite unstable conditions at sea, the clocks remained accurate. Although not as precise as laboratory optical clocks, all three Vector Atomic clocks remained accurate enough to keep time within 400 trillionths of a second, even if access to an external clock was lost for a day.

The Vector Atomic team continues to make smaller, field-stable optical clocks. The researchers predict that the development of such clocks has the potential to reduce maritime reliance on GPS and also open opportunities for scientists to conduct high-precision studies at sea. (J. D. Roslund et al., Nature 628, 736, 2024.)

Close Modal

or Create an Account

Close Modal
Close Modal