  Many proposals for quantum information processing are subject to detectable
loss errors. In this paper, we show that topological error correcting codes,
which protect against computational errors, are also extremely robust against
losses. We present analytical results showing the maximum tolerable loss rate
is 50 %, which is determined by the square-lattice bond percolation threshold.
This saturates the bound set by the no-cloning theorem. Our numerical results
support this, and show a graceful trade-off between computational and loss
errors.
