  The clock synchronization problem is to determine the time difference T
between two spatially separated parties. We improve on I. Chuang's quantum
clock synchronization algorithm and show that it is possible to obtain T to n
bits of accuracy while communicating only one qubit in one direction and using
an O(2^n) frequency range. We also prove a quantum lower bound of \Omega(2^n)
for the product of the transmitted qubits and the range of frequencies, thus
showing that our algorithm is optimal.
