  Precision navigation and timing, very-long-baseline interferometry,
next-generation communication, sensing, and tests of fundamental physics all
require a highly synchronized network of clocks. With the advance of
highly-accurate optical atomic clocks, the precision requirements for
synchronization are reaching the limits of classical physics (i.e. the standard
quantum limit, SQL). Efficiently overcoming the SQL to reach the fundamental
Heisenberg limit can be achieved via the use of squeezed or entangled light.
Although approaches to the Heisenberg limit are well understood in theory, a
practical implementation, such as in space-based platforms, requires that the
advantage outweighs the added costs and complexity. Here, we focus on the
question: can entanglement yield a quantum advantage in clock synchronization
over lossy satellite-to-satellite channels? We answer in the affirmative,
showing that the redundancy afforded by the two-mode nature of entanglement
allows recoverability even over asymmetrically lossy channels. We further show
this recoverability is an improvement over single-mode squeezing sensing,
thereby illustrating a new complexity-performance trade-off for space-based
sensing applications.

Quantum Physics