Performance of a High-Performance Optical Atomic Clock in the Navier-Ship Environment: The VIPER Clock
The operating environment on the ship differed greatly from NIST, but the clock still ran with high performance. 1c,d). The Conex was air-conditioned but the internal environment had swings in temperature and relative humidity. The clock rack was in front of the air conditioning unit, and it ran on and off throughout the day. Through the ship motion, the clocks operated continuously. The rotational dynamics of the ship included a peak pitch of ±1.5° at a rate of ±1.2° s−1 and a peak roll of ±6° at a rate of ±3° s−1. Similarly, the maximum surge, sway and heave accelerations were ±0.4, ±1.5 and ±1.2 m s−2, respectively. A vertical root mean square vibration of 0.03 m s−2 (integrated from 1 to 100 Hz) was also experienced. Operation in dynamic environments highlights the robust, high-bandwidth clock readout (greater than 10 kHz control bandwidth) enabled by a vapour cell.
Atomic timekeeping plays an essential role in modern infrastructure, from transportation to telecommunications to cloud computing. Billions of devices rely on the Global Navigation Satellite System for accurate positioning and synchronization11. The Global navigation satellite system is a network of distributed high- performance microwave-based atomic clocks. The emergence of fieldable optical timekeeping, which offers femtosecond timing jitter at short timescales and multiday, subnanosecond holdover, along with long-distance femtosecond-level optical time transfer12, paves the way for global synchronization at picosecond levels.
There is a short-term instability of 1.3 1013/(sqrttau ) and a more prominent diurnal temperature instability that peaks at 4 1014 near 40,000 s. The VIPER physics package is an earlier design with relaxed performance goals that results in a larger temperature coefficient than the other two clocks. Nonetheless, this system can average over the diurnal temperature fluctuation and maintain an instability of 2.5 × 10−14 after 1 day of averaging. During the underway, a drift rate was similar to the ones shown by PICKLES and EPIC. Importantly, the VIPER physics package does not include magnetic shields yet still provides excellent frequency stability despite motion through Earth’s magnetic field.
There is a broad feature with a peak deviation of four 1015 in the PICKLES Allan deviation. The optical frequencies deviation of 2 Hz correspond to a shift of 2 parts per million at the hyperfine transition line centre. We suspect that the origin of this plateau in PICKLES is RAM coupling through a spurious etalon in the spectrometer. 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, as reflected in the benefits of optical frequency division.
Colocated at-sea testing of three iodine and strontium clocks for Earth’s magnetic field and magnetic field mapping
The vessel travelled in all four cardinal directions during the exercise, illustrated by the GPS-tracked trajectory in Fig. 3c. The projection of Earth’s magnetic field at this latitude and longitude varied by 270 mG throughout the workday, according to a model created by the National Ocean and Atmospheric Administration.
All three clocks were colocated for the at-sea testing; therefore, there is potential for correlated environmental sensitivities due to ship dynamics, motion in Earth’s magnetic field and temperature and humidity variations inside the Conex. Standard reference clocks (such as a caesium beam clock or GPS-disciplined rubidium) were not available for comparison. The amount of common mode rejection needed to mask fluctuations from the three systems was raised by the simultaneous evaluation of three clocks. The at-sea test data of three clocks is combined with environmental testing on land to provide confidence in potential correlations.
Scientists have figured out a way to slice time even more finely by using elements like strontium that light up at higher frequencies. The optical clocks are usually the size of dining tables and operate under laboratory controlled conditions.
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. But the clock is much more robust and around one 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. The molecule is less sensitive than some atoms to temperature fluctuations, Magnetic fields, and pressure, according to the physicist Martinboyd co-author of the paper.
Future models could fly aboard global navigation satellites if the team can shrink the clock further. They could even be the clocks that end up defining lunar time, he says.
It is possible to improve research by using a portable clock that is precise, such as mapping Earth’s gravitational field or using multiple telescopes to image black holes.
If navigation signals are spoofed and jammed in conflict zones, the Space-Time Standards Laboratory of the Radio Research Institute could provide a “vital fallback solution” by using the clock.
Elizabeth Donley is the time and Frequency division head at the US National Institute of Standards and Technology. We are excited to get our hands on it.