On the detection of single mode quantum coherence at optical frequencies
Protocols discussed in the context of quantum information theory often require the ability to implement preparation procedures associated with one of several nonorthogonal density operators, and measurement procedures associated with one of several non-orthogonal bases. A single mode of the electromagnetic field at optical frequencies might at first appear to be a candidate system for implementing such tasks. The quantum state of a laser above threshold is typically assumed to be well approximated by a coherent state, and any two coherent states are strictly non-orthogonal. Moreover, optical homodyne tomography would appear to provide a way of measuring (at least upon repeated preparations) an arbitrary quadrature of the field. However, we shall argue that conventional preparation and measurement procedures on single mode optical fields are not appropriate for quantum information processing.