  We study condensation of abelian bosons in string-net models, by constructing
a family of Hamiltonians that can be tuned through any such transition. We show
that these Hamiltonians admit two exactly solvable, string-net limits: one deep
in the uncondensed phase, described by an initial, uncondensed string net
Hamiltonian, and one deep in the condensed phase, described by a final,
condensed string net model. We give a systematic description of the condensed
string net model in terms of the uncondensed string net and the data associated
with the condensing abelian bosons. Specifically, if the uncondensed string net
is described by a fusion category $\mathcal{C}$, we show how the string labels
and fusion data of the fusion category $\mathcal{\tilde{C}}$ describing the
condensed string net can be obtained from that of $\mathcal{C}$ and the data
describing the string oeprators that create the condensing boson. This
construction generalizes previous approaches to anyon condensation in string
nets, by allowing the condensation of arbitrary abelian bosons, including
chiral bosons in string nets constructed from (for example) Chern-Simons
theories, which describe time-reversal invariant bilayer states. This gives a
method for obtaining the full data for string nets without explicit
time-reversal symmetry from such bilayer models. We illustrate our approach
with several examples.
