Time synchronization in an array of optical telescopes: tagging photons with pico-second accuracy

About

High-precision (nanosecond- to picosecond-scale) time synchronization in astronomy is used mainly in radio interferometry (e.g., VLBI) and Imaging Atmospheric Cherenkov Telescopes (IACTs), which time-tag air showers from cosmic gamma rays. In contrast, intensity interferometry in the visible requires stricter timing, as correlations of intensity fluctuations measured across telescope baselines are used to infer source sizes.

Michelson-type interferometers (e.g., CHARA, VLTI) are limited to baselines of a few hundred meters due to optical delay lines technical complexity, whereas intensity interferometry—being fully digital—can operate over much longer baselines, limited primarily by time synchronization. Atomic clocks with picosecond-level drift could enable global-scale baselines, though practical systems are expected to span a few to tens of kilometers using fiber-based synchronization technologies.

White Rabbit (WR), developed at CERN and based on the Precision Time Protocol, provides sub-nanosecond synchronization over commercial single-mode fibers. We show that WR can achieve stable synchronization at the few-picosecond level over kilometer distances for systems with multiple clients and central master clock. The main limitation is temperature-induced fiber delay variation. Measured performance shows 9 ps RMS jitter for realistic temperature change profiles, supporting the feasibility of WR for long-baseline intensity interferometry.

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