Abstract
Projection operators associated with antiunitary time-reversal symmetry differ from the usual orthogonal projections encountered in Hilbert space. Operators that act in Fock space and project parts of a single-mode state of the electromagnetic field that can be identified as its quadratures are defined and their properties are verified by consideration of their action on single-mode coherent states. The fact that their construction involves, in an essential way, not one but two parameters is discussed and a weakened form of orthogonal projection that is specialized to a particular Fock state superposition is identified. The question of whether or not such orthogonal projections are universal is raised. In passing, it is noted that the possibility of classifying states as well as operators according to their time-reversal symmetry properties is relevant to the analysis of nonlinear wave interactions between waves with rationally related frequencies.
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