  Modular mixed-species ion-trap networks are a promising framework for
scalable quantum information processing, where one species acts as a memory
qubit and another as a communication qubit. This architecture requires
high-fidelity mixed-species entangling gates to transfer information from
communication to memory qubits through their collective motion. We investigate
the character of the motional modes of a mixed-species ion chain for entangling
operations and also sympathetic cooling. We find that the laser power required
for high-fidelity entangling gates based on transverse modes is at least an
order of magnitude higher than that based on axial modes for widely different
masses of the two species. We also find that for even moderate mass
differences, the transverse modes are much harder to cool than the axial modes
regardless of the ion chain configuration. Therefore, transverse modes
conventionally used for operations in single-species ion chains may not be well
suited for mixed-species chains with widely different masses.
