Optical Two-Way Time-Frequency Transfer Across A Three-Node Free-Space Network

We demonstrate optical two-way time-frequency transfer (O-TWTFT) across a three-node free-space network. The network spans 28 km of outdoor air and relays time and frequency from a master node, through an intermediate fielded node, to an end node. Despite strong atmospheric turbulence along the close-to-ground free-space links and large ambient temperature swings at the fielded node, the network transfers time with relative timing offsets below 5 fs. Additionally, the network transfers frequency with instability below 10(-18) at averaging times greater than 200 s-i.e. below the level needed to compare state-of-the-art optical atomic clocks.