A clock network for geodesy and fundamental science (1511.07735v1)

Abstract: Leveraging the unrivaled performance of optical clocks in applications in fundamental physics beyond the standard model, in geo-sciences, and in astronomy requires comparing the frequency of distant optical clocks truthfully. Meeting this requirement, we report on the first comparison and agreement of fully independent optical clocks separated by 700 km being only limited by the uncertainties of the clocks themselves. This is achieved by a phase-coherent optical frequency transfer via a 1415 km long telecom fiber link that enables substantially better precision than classical means of frequency transfer. The fractional precision in comparing the optical clocks of three parts in $10^{17}$ was reached after only 1000 s averaging time, which is already 10 times better and more than four orders of magnitude faster than with any other existing frequency transfer method. The capability of performing high resolution international clock comparisons paves the way for a redefinition of the unit of time and an all-optical dissemination of the SI-second.

• The paper demonstrates a breakthrough in remote optical clock comparisons over a 700 km link achieving a 3×10^-18 fractional precision within 1000 s.
• The methodology utilizes strontium lattice clocks and femtosecond frequency combs to enable direct optical link comparisons, bypassing microwave limitations.
• The results promise advancements in geodesy and fundamental science, paving the way for redefining the SI-second and monitoring continental-scale height changes.

A Clock Network for Geodesy and Fundamental Science

This paper presents a significant advancement in the domain of optical frequency metrology by demonstrating the first high-precision comparison of fully independent optical clocks situated 700 km apart. These optical frequency comparisons were conducted through a 1415 km long phase-coherent optical frequency transfer via a telecom fiber link. The resulting fractional precision achieved was three parts in $10^{18}$ after just 1000 seconds of averaging time. This represents a tenfold improvement in precision and a reduction in averaging duration by over four orders of magnitude when compared to existing frequency transfer methodologies.

The research employs strontium lattice clocks located at two national metrology institutes: LNE-SYRTE in Paris, France, and PTB in Braunschweig, Germany. Through the use of femtosecond frequency combs, the local clock laser’s frequency is accurately represented against a transfer laser, which transmits the frequency information along stabilized fiber links connecting the two cities via a midpoint in Strasbourg. This setup allows for direct optical clock comparison, bypassing the reliance on cesium microwave clocks or satellite-based methods, which have limited accuracy.

Optical clocks, with significantly enhanced precision over microwave clocks, are poised to revolutionize various fields, from fundamental physics and geodesy to astronomy. This research exemplifies the potential to investigate phenomena such as the search for dark matter through localized time scale jitter, perform rigorous tests of the Lorentz invariance, and enhance very long baseline interferometry synchronization. Moreover, optical clocks can be instrumental in the new geodetic reference frames' development and foster the evolution of a quantum network of clocks.

With a fractional clock uncertainty reaching $4 \times 10^{-17}$ in Paris and $2 \times 10^{-17}$ in Braunschweig, and systematic uncertainties adequately adjusted for all known shifts and biases, the comparison between these clocks paves the way for a redefinition of the SI-second. This marks a substantial step forward, moving beyond the capabilities afforded by existing microwave clock comparisons.

The experimental setup demonstrates the capacity to carry out high-resolution international clock comparisons. The integrated link system encountered notably low uncertainty contributions, achieving unprecedented precision levels for remote clock measurements. For practicing geodesy and other spatiotemporal science applications, such links offer the precision and bandwidth necessary for monitoring centimeter-level height changes over continental expanses.

As the next step, further integration of additional institutes into this continental-scale optical clock network is anticipated. This will unlock even greater precision, enabling researchers to push the bounds of geodetic metrics and height-reference frameworks through clock-based methods. The long-term vision extends towards redefining the second based upon the enhanced stability and precision of optical atomic clocks, as foreseen by metrology experts.

The implications for future developments in atmospheric, space-time, and quantum sciences are profound, with the potential to refine measurement standards and redshift tests, and assess time-dependent phenomena with unparalleled accuracy. This research establishes a fundamental platform for future explorations and applications in precision metrology.