Quantum algorithm for distributed clock synchronization

The clock synchronization problem is to determine the time difference Δ between two spatially separated clocks. When message delivery times between the two clocks are uncertain, O(22n) classical messages must be exchanged between the clocks to determine n digits of Δ. On the other hand, as we show, there exists a quantum algorithm to obtain n digits of Δ while communicating only O(n) quantum messages. © 2000 The American Physical Society.