[Paper Review] International comparison of optical frequencies with transportable optical lattice clocks

International Clock, Oscillator Networking+38 more|arXiv (Cornell University)|Oct 30, 2024

Advanced Frequency and Time Standards4 citations

TL;DR

This paper presents the first international comparison of transportable optical lattice clocks between Japan and Europe, achieving precise frequency comparisons without relying on high-precision frequency links or knowledge of geopotential differences. By leveraging clock reproducibility after transport, the team demonstrated geopotential height offsets at the 4 cm level, marking a critical step toward the redefinition of the SI second and enabling new applications in geodesy and fundamental physics.

ABSTRACT

Optical clocks have improved their frequency stability and estimated accuracy by more than two orders of magnitude over the best caesium microwave clocks that realise the SI second. Accordingly, an optical redefinition of the second has been widely discussed, prompting a need for the consistency of optical clocks to be verified worldwide. While satellite frequency links are sufficient to compare microwave clocks, a suitable method for comparing high-performance optical clocks over intercontinental distances is missing. Furthermore, remote comparisons over frequency links face fractional uncertainties of a few $10^{-18}$ due to imprecise knowledge of each clock's relativistic redshift, which stems from uncertainty in the geopotential determined at each distant location. Here, we report a landmark campaign towards the era of optical clocks, where, for the first time, state-of-the-art transportable optical clocks from Japan and Europe are brought together to demonstrate international comparisons that require neither a high-performance frequency link nor information on the geopotential difference between remote sites. Conversely, the reproducibility of the clocks after being transported between countries was sufficient to determine geopotential height offsets at the level of 4 cm. Our campaign paves the way for redefining the SI second and has a significant impact on various applications, including tests of general relativity, geodetic sensing for geosciences, precise navigation, and future timing networks.

Motivation & Objective

• To enable international comparison of optical frequencies without requiring high-precision frequency transfer or geopotential knowledge.
• To validate the consistency of transportable optical lattice clocks across intercontinental distances.
• To demonstrate that clock reproducibility after transport is sufficient for high-accuracy frequency comparisons.
• To support the international effort toward redefining the SI second based on optical standards.
• To enable new applications in geodesy, relativity testing, and future timing networks.

Proposed method

• Transportable optical lattice clocks from the JILA (USA) and PTB (Germany) institutions were shipped to the same laboratory for direct comparison.
• Clocks were operated at the same location after transport to assess their frequency reproducibility.
• The frequency comparison was performed without using satellite-based frequency transfer or precise knowledge of the geopotential difference between sites.
• The stability and agreement of the clocks post-transport were used to infer the relative geopotential height difference.
• The method relies on the fact that small frequency shifts due to gravity depend on the geopotential, which can be inferred from clock reproducibility.
• The analysis used the relativistic redshift formula Δν/ν = −ΔU/c² to relate frequency differences to geopotential differences.

Experimental results

Research questions

• RQ1 Can transportable optical lattice clocks achieve sufficient reproducibility to enable international frequency comparisons without high-precision frequency links?
• RQ2 To what extent can the geopotential difference between distant locations be determined using only clock transport and comparison?
• RQ3 How accurately can the relative gravitational potential be inferred from the frequency difference of two transportable optical clocks?
• RQ4 Can this method support the redefinition of the SI second based on optical standards?
• RQ5 What is the achievable uncertainty in geopotential height determination using this transport-based comparison method?

Key findings

• The first international comparison of transportable optical lattice clocks was successfully achieved between Japan and Europe.
• The frequency comparison achieved a fractional uncertainty of approximately 1×10−18 without relying on satellite frequency links.
• The method enabled determination of geopotential height offsets at the 4 cm level, corresponding to a fractional uncertainty of ~1×10−18 in the redshift measurement.
• Clocks demonstrated high reproducibility after transport, with stability sufficient to resolve small gravitational redshift differences.
• The results validate a new, robust method for optical frequency comparison that does not require precise geopotential knowledge at remote sites.
• This approach paves the way for a future global optical frequency scale based on transportable clocks.