Demonstration of a mobile optical clock ensemble at sea

Abstract

Atomic clocks are at the leading edge of accuracy and precision and are essential for synchronisation of distributed critical infrastructure, position, navigation and timing, and scientific applications. There has been significant improvements in the performance of atomic clocks with the shift from microwave to optical frequency transitions. However, this performance increase has come at the cost of size, complexity and fragility, which has confined optical clocks to laboratories. Here we report on a recent international collaboration where three emerging optical clocks, each based on different operating principles, were trialled at sea. These clocks demonstrated world-class performance and reliability by providing stable frequency outputs in optical, microwave and radio-frequency domains over three weeks of unsupervised naval operation.

Fig. 1. Portable optical clocks and trial route.

a The mobile clock systems: the UofA Yb clock, the UofA Rb clock, and the AFRL Optical Rubidium Atomic Frequency Standard (ORAFS). b The route taken by the HMNZS Aotearoa during one of the field trials, with an inset showing loading of the Office of Naval Research (ONR) SeaBox onto the deck of the ship via the ship’s crane. Made with Natural Earth. Free vector and raster map data @ naturalearthdata.com.

Fig. 2. Clock comparison scheme.

Schematic of the comparison system used to extract the RF and optical frequency stability of each clock, with RF clock outputs (black), microwave outputs (grey), optical outputs (purple) and down-mixed beat notes (dashed).

Fig. 3. Clock output frequency timeseries.

Frequency vs time measurement of the 100 MHz RF outputs of the three clocks directly measured by the frequency counter during different stages of the trial. The three systems have been offset by 200 × 10⁻¹² for clarity. The ORAFS data has been processed to remove shifts between data segment due to changes in the comb mode index used for optical to RF down-conversion. An additional offset of 2 × 10⁻⁹ has been removed from three segments of ORAFS data in which the clock was operated in a different locking scheme — denoted by a box with star (*).

Fig. 4. Measurement reference acceleration sensitivity.

a A two minute section of the fractional frequency shift of the optical beats between the three clocks measured by the frequency counter referenced to the 5071A. b The fractional frequency shifts of the 100 MHz outputs of each optical clock measured directly by the frequency counter referenced to the 5071A. c Vertical acceleration measured by a sensor within the clock.

Fig. 5. Clock stabilities in pre-trial location.

Synthesised individual clock Modified Allan deviations (ModADEVs) calculated for the three direct optical comparisons (solid lines) and synthesised ModADEVs from 100 MHz measurements (dashed lines) while the clocks were on shore. 5071A counter reference is shown in black dashed line with closed circles. Shaded regions indicate the statistical confidence interval, as described in ref. . Use of the three-cornered hat method allows extraction of the performance of each individual clock, without noise or drift contributions from the 5071A counter reference. As discussed in text, the 100 MHz measurements are limited by counter noise, while the direct 1 GHz UofA Yb and UofA Rb clock comparison, as well as the three direct inter-clock optical comparisons, are not limited by this noise source.

Fig. 6. Clock stabilities at sea.

Synthesised individual clock Modified Allan deviations (ModADEVs) calculated for the three direct optical comparisons (solid lines) and synthesised ModADEVs from 100 MHz measurements (dashed lines) while the clocks were at sea. 5071A counter reference is shown in black dashed line with closed circles. Shaded regions indicate the statistical confidence interval, as described in ref. .

Fig. 7. SWAP comparison between existing clock systems.

Fractional frequency stability at 1 s vs Size-Weight-and-Power of UofA Yb, UofA Rb and ORAFS, compared to a limited selection of commercially available and emerging microwave (red) and optical (blue) portable atomic clocks. CSAC: Microchip Chip scale atomic clock; PRS 10: Stanford Research Systems Low phase noise Rb oscillator; 5071A: Microchip 5071A Primary frequency standard; SD cRb: SpectraDynamics cold Rubidium microwave atomic clock; MuClock: Muquans MuClock; MHM Maser: Microchip MHM-2020 Active hydrogen maser; DSAC: Deep space atomic clock; NPL Cs: National Physics Laboratory Cs Fountain; VA: Vector Atomic Evergreen-30; OptiClock: Yb ion clock; Tiqker: Infleqtion Tiqker; WIPM 2020: Ca ion clock; LENS: Sr Lattice clock; RIKEN: Sr Lattice clock; PTB: Sr Lattice clock. This has been reproduced from ref. .