  Motivated by the desire to construct meson-meson operators of definite
relative momentum in order to study resonances in lattice QCD, we present a set
of single-meson interpolating fields at non-zero momentum that respect the
reduced symmetry of a cubic lattice in a finite cubic volume. These operators
follow from the subduction of operators of definite helicity into irreducible
representations of the appropriate little groups. We show their effectiveness
in explicit computations where we find that the spectrum of states interpolated
by these operators is close to diagonal in helicity, admitting a description in
terms of single-meson states of identified J^{PC}. The variationally determined
optimal superpositions of the operators for each state give rapid relaxation in
Euclidean time to that state, ideal for the construction of meson-meson
operators and for the evaluation of matrix elements at finite momentum.
