  We stabilize a chosen radiofrequency beat note between two optical fields
derived from the same mode-locked laser pulse train, in order to coherently
manipulate quantum information. This scheme does not require access or active
stabilization of the laser repetition rate. We implement and characterize this
external lock, in the context of two-photon stimulated Raman transitions
between the hyperfine ground states of trapped 171-Yb+ quantum bits.
