Session P2a: Advanced and Future Clocks

Iodine Optical Clock use at the Event Horizon Telescope for VLBI
M. Ledbetter, A. Kowligy, J. Roslund, A. Cingoz, G. Partridge, P. Patel, E. Pashollari, E. Popp, F. Roller, D. Sheredy, G. Skulason, J. Song, E. Atchison, O. Husain, P. Carney, MK Pasha, A. Rakholia, A. Dowd, J. Abo-Shaeer, M. Boyd, Vector Atomic; D. Marrone, G. Reiland, Department of Astronomy/Steward Observatory
Location: Seaview A/B
Date/Time: Wednesday, Jan. 29, 9:43 a.m.

We report on use of iodine clocks for very long baseline interferometry (VLBI) applications and demonstration of Vector Atomic’s EG30 optical clock at the Event Horizon Telescope[1] during a recent science mission.
Iodine clocks employ a robust vapor cell architecture that provides excellent short-term instability, uses no consumables, requires no laser cooling or trapping, no cavity or vacuum subsystems, and is insensitive to platform motion. Environmental systematics such as magnetic fields, light shifts and collisions are relaxed in comparison to alkali vapor cell clocks. The approach enables high performance to be maintained in field applications including mobile clocks [2] and all optical picosecond timing networks [3,4].
Characterization of EG30 clock metrics relevant to VLBI both in stable laboratory environments and while installed in the field at a remote observatory will be presented. Short term instability of < 3 x 10^-14 at 1 s, integrating down into the mid 10^-16s at 10^4 s is demonstrated when used in stable environments. While installed at the Arizona Radio Observatory on Kitt Peak, EG30 exhibits superior performance to state-of-the-art masers over target integration windows from 0.01 to 100 s, potentially enabling new capabilities in radio astronomy. The clock was then used in the spring 2024 campaign for the Event Horizon Telescope collaboration. Interference fringes were obtained while observing quasar 3c279[4] using EG30 in the 225 GHz microwave chain. We will present these results as well as other recent field demos with iodine clocks.
1. EHT collaboration, “Event Horizon Telescope imaging of the archetypal blazar 3C 279 at an extreme 20 micro-arcsecond resolution”, Astro. & Astrophys Vol 640, 2020
2. Jonathan Roslund et. al, “Optical clocks at Sea,” Nature 628, 736-740 (2024).
3. A. Kowligy et al., "Multinode optical clock network linked via quantum-limited two-way time transfer," IEEE International Frequency Control Symposium (IFCS), Taipei, Taiwan, 2024
4. Jonathan Roslund et. al, “Optical two-tone time transfer,” arXiv:2408.09290 (2024)