Session P3a: Low-SWaP Clocks and Oscillators

Deployed Optical Time Transfer at the Femtosecond Level
Jonathan D. Roslund, Abijith S. Kowligy, Junichiro Fujita, Micah P. Ledbetter, Evan Popp, Frank Roller, Daniel B. Sheredy, Evan Atchinson, Elton Pashollari, Akash V. Rakholia, Gunnar Skulason, Andrew Dowd, Martin M. Boyd, and Jamil R. Abo-Shaeer, Arman Cingoz, Vector Atomic; Emily Caldwell, Fabrizio Giorgetta, Thea Triano, Bill Swann, and Laura Sinclair, NIST; Jean-Daniel Deschenes, Octosig
Location: Seaview A/B
Alternate Number 1

Sub-picosecond timing synchronization can enable future optical timekeeping networks for applications including coherent phased array radar imaging at GHz levels, intercontinental clock comparisons for the redefinition of the second, chronometric leveling, and synchronization of remote assets, including future satellite-based optical time standards. With optical clocks now operating on mobile platforms [1,2], free-space synchronization strategies with compatible performance are essential to expand the reach of precision timing. Vector Atomic is developing synchronization hardware to support such timing networks; here we report on two efforts.
First, we constructed a three-node timing network using portable iodine clocks connected over a free-space link using comb-based optical two-way time transfer [3]. Two rackmount iodine clocks (A and B) supporting ~5x10-14/?t instability operated in a common laboratory at NIST Boulder. A third rackmount iodine clock (C) supporting ~2.5x10-14/?t [4] was located at a remote facility 15 km away on Table Mountain. Each clock was paired with a rackmount synchronization transceiver, consisting of two self-referenced Er:fiber frequency combs and a low-loss photonic integrated circuit. Timing differences were measured over-the-air between clocks A-C and B-C, providing a three-way timing comparison over multiple days without user intervention. Agreement was observed between the A-B temporal deviations retrieved over 30 km of air via the intermediate clock at B and the locally measured A-B time variations. The simultaneous deployment of three optical clocks and nine frequency combs demonstrates the viability of future optical timing networks.
Next, we introduce a simplified time transfer scheme with comparable performance that reduces hardware requirements and can support both fiber and free-space networks [5]. This technique replaces the frequency comb transmitted across the link with two CW lasers and retrieves the time-of-flight via a spectral projective technique. The use of two CW lasers in place of a comb reduces the SWaP-C of the transceiver, increases the dynamic range by 1,000x verses existing CW techniques, and alleviates complications associated with dispersion-induced broadening of femtosecond pulses. Using this technique over a ?100 m free-space link, synchronization below 400 attoseconds at one second of averaging was demonstrated and maintained below 1 fs for several hours. In addition, the link was used to syntonize two iodine optical clocks and compare them over four days. The demonstration employed an integrated photonics transceiver and telecom-band lasers that are compatible with full photonic integration.
[1] J.D. Roslund, et al., Optical clocks at sea. Nature, 628(8009), 736-740.
[2] E. Ahern, et al., Demonstration of a Mobile Optical Clock Ensemble at Sea. arXiv:2406.03716.
[3] E.D. Caldwell, et al., Quantum-limited optical time transfer for future geosynchronous links. Nature, 618(7966), 721-726.
[4] https://vectoratomic.com/eg30-advanced-release
[5] J.D. Roslund, et al., Optical Two-Tone Time Transfer. arXiv:2408.09290.