  We describe a nuclear magnetic resonance (NMR) experiment which implements an
efficient one-to-two qubit phase-covariant cloning machine(QPCCM). In the
experiment we have achieved remarkably high fidelities of cloning, 0.848 and
0.844 respectively for the original and the blank qubit. This experimental
value is close to the optimal theoretical value of 0.854. We have also
demonstrated how to use our phase-covariant cloning machine for quantum
simulations of bit by bit eavesdropping in the four-state quantum key
distribution protocol.
