  In general, fault-tolerant quantum error correction (FTQEC) procedures are
designed to detect, correct, and be fault-tolerant against errors occurring
within the qubit subspace. But in some qubit implementations, additional
"leakage" errors can occur in which the system leaves this subspace, and
standard FTQEC procedures may not be fault-tolerant against such errors.
Generic methods for achieving fault-tolerance against leakage are costly in
terms of resources. In this paper we demonstrate that for a leakage model
common to many photonic gate implementations, FTQEC can be implemented with far
fewer additional operations than in the generic case.
