  We demonstrate carrier-phase optical two-way time-frequency transfer
(carrier-phase OTWTFT) through the two-way exchange of frequency comb pulses.
Carrier-phase OTWTFT achieves frequency comparisons with a residual instability
of 1.2x10$^{-17}$ at 1 second across a turbulent 4-km free space link,
surpassing previous OTWTFT by 10-20x and enabling future high-precision optical
clock networks. Furthermore, by exploiting the carrier-phase, this approach is
able to continuously track changes in the relative optical phase of distant
optical oscillators to 9 mrad (7 attoseconds) at 1-sec averaging, effectively
extending optical phase coherence over a broad spatial network for applications
such as correlated spectroscopy between distant atomic clocks.
