Quantum algorithm for distributed clock synchronization
  The clock synchronization problem is to determine the time difference $\Delta$ between two spatially separated clocks. When message delivery times between the two clocks are uncertain, $O(2^{2n})$ classical messages must be exchanged between the clocks to determine $n$ digits of $\Delta$. On the other hand, as we show, there exists a quantum algorithm to obtain $n$ digits of $\Delta$ while communicating only O(n) quantum messages. 
Isaac Chuang