On Decoherence in Quantum Clock Synchronization
  We study two quantum versions of the Eddington clock-synchronization protocol in the presence of decoherence. The first protocol uses maximally entangled states to achieve the Heisenberg limit for clock synchronization. The second protocol achieves the limit without using entanglement. We show the equivalence of the two protocols under any single-qubit decoherence model that does not itself provide synchronization information. 
Sergio Boixo, Carlton M. Caves, Animesh Datta, Anil Shaji