Abstract
We demonstrate frequency-comb-based optical two-way time-frequency transfer across a three-node clock network. A fielded, bidirectional relay node connects laboratory-based master and end nodes, allowing the network to span 28 km of turbulent outdoor air while keeping optical transmit powers below 5 mW. Despite the comparatively high instability of the free-running local oscillator at the relay node, the network transfers frequency with fractional precision below 10−18 at averaging times above 200 s and transfers time with a time deviation below 1 fs at averaging times between 1 s and 1 h. The successful operation of this network represents a promising step toward the operation of future free-space networks of optical atomic clocks.
Highlights
Optical clock networks hold great promise for a wide range of applications.1–16 For example, networked clocks could aid the search for dark matter,3–6 enable tests of relativity,3,7–9 or provide the timebases necessary for very long baseline interferometry.10 networked optical clocks are required to probe the earth via relativistic geodesy11–15 and to meet the criteria for the redefinition of the SI second.16 For clock networks to reach their full potential, high-precision methods of transferring time and frequency between clocks must be developed, for fiber links and for free-space links
As described in Refs. 18 and 21, optical two-way time-frequency transfer (O-TWTFT) is used for both time and frequency transfer between clocks
The frequency-transfer performance of O-TWTFT may be deduced from its time-transfer performance, we consider the two cases separately for two reasons
Summary
Optical clock networks hold great promise for a wide range of applications. For example, networked clocks could aid the search for dark matter, enable tests of relativity, or provide the timebases necessary for very long baseline interferometry. networked optical clocks are required to probe the earth via relativistic geodesy and to meet the criteria for the redefinition of the SI second. For clock networks to reach their full potential, high-precision methods of transferring time and frequency between clocks must be developed, for fiber links and for free-space links. Comb-based O-TWTFT over turbulent air paths has been shown to support frequency comparisons below 10−18 21 and to enable full femtosecond time synchronization, even in the presence of motion.22,23 The performance of this optical technique shows many orders of magnitude improvement over state-of-theart, rf-based approaches to time and frequency transfer through free-space.. The network has the topology needed to connect ultra-precise, laboratory-bound clocks that are separated by great distances or that lack line-of-sight connecting paths This network transfers frequency with a fractional instability below 10−18 at a 200-s averaging time and transfers time with a time deviation below 1 fs at averaging times between 1 s and 1 h. Each “optical clock” consists of a self-referenced fiber frequency comb phase-locked to a 1535-nm cavity-stabilized laser that serves as an optical oscillator. At this level of performance, comb-based O-TWTFT could support future deployed clock networks that connect high-performance clocks via lower-performance, fielded relay nodes
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