The atomic clock is on a rocking naval ship
High-Performance Molecular Iodine Optical Calorimeters for Space Missions: PICKLES, EPIC and VIPER
Molecular iodine (I2) has a legacy as an optical frequency standard13,14,15,16,17. The species underpinned one of the first demonstrations of optical clocks, as it was officially recognized as length standards. More recently, iodine frequency standards have been investigated for space missions21,22,23,24. 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.
Normally, an atomic clock is either sturdy or ultra-precise. Now, scientists have created a precise clock that, when put through its paces aboard a naval ship, wavered by only 300-trillionths of a second per day.
The temperature peaks at a time of over 40,000 s with a short-term instability of 1.3 1013/. The VIPER physics package is an earlier design with relaxed performance goals that results in a larger temperature coefficient than the other two clocks. The average temperature of this system can be adjusted to maintain a 2.5 1014 stability after the first day of averaging. The drift rate was very similar to 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.
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 suspect that the origin of this plateau in PICKLES is RAM coupling through a spurious etalon in the spectrometer. The build procedure was changed to make sure this eta can be mitigated.
The iodine clock has excellent phase noise for 10 and 100 MHz tones, derived by optical frequency division, and a 1,064 nm optical output. The phase noise at microwaves is lower than atomic-disciplined oscillators, which can be used to transfer the noise of the laser to the frequencies.
Simultaneous Evaluation of a Clock for Mapping Earth’s Magnetic Field and Temperature: A Correlated Molecular Atomic Clock
The vessel travelled in all four cardinal directions during the exercise, illustrated by the GPS-tracked trajectory in Fig. 3c. The model for the Earth’s magnetic field at this latitude and temperature shows a different projection of the earth’s field on the clock throughout the day.
The potential of correlated environmental sensitivities can be found 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. However, simultaneous evaluation of three clocks raises the level of common mode rejection required to mask fluctuations common to the three systems, particularly given VIPER’s differing spectrometer and laser system designs. The correlations between the at-sea test data and the measured instability are shown in theSupplementary Information.
The clock tick is based on the radiation that atoms absorb and emit as they change energy states. Caesium and other elements have been used in clock-making for a long time. Some are portable and are sold commercially.
Donley thinks that it is similar to a hydrogen maser clock, a reliable kind of microwave atomic clock that is the workhorse for international timekeeping. The clock is not as strong as it could be, but it is 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. The molecules are less sensitive than some atoms to temperature fluctuations, magnetic fields, and pressure according to the paper authored by physicist Martin Boyd.
If the team can shrink the clock further, future models could fly aboard global navigation satellites, improving positioning resolution from metres to centimetres, adds Abo-Shaeer. He says that the clock could be the one that defines lunar time.
A portable clock that can be used to improve research that requires precise timing in the field could be used to map Earth’s gravity field and image black holes.
The Space-Time Standards Laboratory of the Radio Research Institute has an example of a clock that can help if navigation signals are spoofed in conflict zones.
Elizabeth Donley is the time and Frequency division director at the USNational Institute of Standards and Technology in Boulder, Colorado. “We’re excited to get our hands on it.”
