  Synchronous laser-microwave networks delivering attosecond timing precision
are highly desirable in many advanced applications, such as geodesy,
very-long-baseline interferometry, high-precision navigation and
multi-telescope arrays. In particular, rapidly expanding photon science
facilities like X-ray free-electron lasers and intense laser beamlines require
system-wide attosecond-level synchronization of dozens of optical and microwave
signals up to kilometer distances. Once equipped with such precision, these
facilities will initiate radically new science by shedding light on molecular
and atomic processes happening on the attosecond timescale, such as
intramolecular charge transfer, Auger processes and their impact on X-ray
imaging. Here, we present for the first time a complete synchronous
laser-microwave network with attosecond precision, which is achieved through
new metrological devices and careful balancing of fiber nonlinearities and
fundamental noise contributions. We demonstrate timing stabilization of a
4.7-km fiber network and remote optical-optical synchronization across a 3.5-km
fiber link with an overall timing jitter of 580 and 680 attoseconds RMS,
respectively, for over 40 hours. Ultimately we realize a complete
laser-microwave network with 950-attosecond timing jitter for 18 hours. This
work can enable next-generation attosecond photon-science facilities to
revolutionize many research fields from structural biology to material science
and chemistry to fundamental physics.
