  In classical Lorentz-invariant field theories, localized soliton solutions
necessarily break translation symmetry. In the corresponding quantum field
theories, the position is quantized and, if the theory is not compactified,
they have continuous spectra. It has long been appreciated that ordinary
perturbation theory is not applicable to continuum states. Here we argue that,
as the Hamiltonian and momentum operators commute, the soliton ground state can
nonetheless be found in perturbation theory if one first imposes that the total
momentum vanishes. As an illustration, we find the subleading quantum
correction to the ground state of the Sine-Gordon soliton.
