  Timescale comparison between optical atomic clocks over ground-to-space and
terrestrial free-space laser links will have enormous benefits for fundamental
and applied science, from measurements of fundamental constants and searches
for dark matter, to geophysics and environmental monitoring. However,
turbulence in the atmosphere creates phase noise on the laser signal, greatly
degrading the precision of the measurements, and also induces scintillation and
beam wander which cause periodic deep fades and loss of signal. We demonstrate
phase stabilized optical frequency transfer over a 265 m horizontal
point-to-point free-space link between optical terminals with active tip-tilt
mirrors to suppress beam wander, in a compact, human-portable set-up. A phase
stabilized 715 m underground optical fiber link between the two terminals is
used to measure the performance of the free-space link. The active optics
terminals enabled continuous, coherent transmission over periods of up to an
hour. We achieve an 80 dB suppression of atmospheric phase noise to
$3\times10^{-6}$ rad$^{2}$Hz$^{-1}$ at 1 Hz, and an ultimate fractional
frequency stability of $1.6\times10^{-19}$ after 40 s of integration. At high
frequency this performance is limited by the residual atmospheric noise after
compensation and the frequency noise of the laser seen through the unequal
delays of the free space and fiber links. Our long term stability is limited by
the thermal shielding of the phase stabilization system. We achieve residual
instabilities below those of the best optical atomic clocks, ensuring
clock-limited frequency comparison over turbulent free-space links.
