  Entangling operations are a necessary tool for large-scale quantum
information processing, but experimental imperfections can prevent current
schemes from reaching sufficient fidelities as the number of qubits is
increased. Here it is shown numerically how multi-toned generalizations of
standard trapped-ion entangling gates can simultaneously be made robust against
noise and mis-sets of the frequencies of the individual qubits. This relaxes
the degree of homogeneity required in the trapping field, making physically
larger systems more practical.
