Photons as qubits

We review the physical principles behind measurement-based quantum information processing with optical fields. The original Knill-Laflamme-Milburn (KLM) scheme, based on single-photon states, is explained, as well as extensions of the scheme inspired by the idea of cluster state quantum computing, which dramatically reduce the resources required for implementation. A careful analysis of the intrinsic error sources in optical cluster state methods indicates that fault-tolerant quantum gates are feasible given developments in current technology. The main impediment to progress is the lack of single-photon sources and we briefly review what kinds of sources are needed. We conclude with a discussion of phase-shift coherent state codes and discuss a possible implementation in circuit quantum electrodynamics.