  I propose a method to remove the axial motional excitation from an ion qubit
within a few oscillation periods of a harmonic trap. The principle is to
prepare another coolant ion in its motional ground state, and then apply a
phonon beam splitter to swap the motional state between the ions without
affecting the internal state. This method requires only the precise control of
the trapping potential, thus its performance does not suffer from the
limitations of current laser-cooling schemes. With sufficient coolant ions
pre-prepared, this method can rapidly re-cool ion qubits during quantum
information processing.
