2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/127579We introduce a scheme for fault tolerantly dealing with losses (or other "leakage" errors) in cluster state computation that can tolerate up to 50% qubit loss. This is achieved passively using an adaptive strategy of measurement - no coherent measurements or coherent correction is required. Since the scheme relies on inferring information about what would have been the outcome of a measurement had one been able to carry it out, we call this "counterfactual" error correction.Published version - much revised and with a new title. Here we now focus solely on the general aspects of the protocol - a much expanded and improved discussion of its application in linear optical quantum computation can now be found in quant-ph/0702044Quantum Physicstext