  The development of precise atomic clocks has led to many scientific and
technological advances that play an increasingly important role in modern
society. Shared timing information constitutes a key resource for positioning
and navigation with a direct correspondence between timing accuracy and
precision in applications such as the Global Positioning System (GPS). By
combining precision metrology and quantum networks, we propose here a quantum,
cooperative protocol for the operation of a network consisting of
geographically remote optical atomic clocks. Using non-local entangled states,
we demonstrate an optimal utilization of the global network resources, and show
that such a network can be operated near the fundamental limit set by quantum
theory yielding an ultra-precise clock signal. Furthermore, the internal
structure of the network, combined with basic techniques from quantum
communication, guarantees security both from internal and external threats.
Realization of such a global quantum network of clocks may allow construction
of a real-time single international time scale (world clock) with unprecedented
stability and accuracy.
