  In biorthogonal quantum mechanics, the eigenvectors of a quasi-Hermitian
operator and those of its adjoint are biorthogonal and complete and the
probability for a transition from a quantum state to any one of these
eigenvectors is positive definite. We apply this formalism to the long standing
problem of the position observable in quantum field theory. The dual bases are
positive and negative frequency one-particle states created by the field
operator and its conjugate and biorthogonality is a consequence of their
commutation relations. In these biorthogonal bases the position operator is
covariant and the Klein-Gordon wave function is localized. We find that the
invariant probability for a transition from a one-photon state to a position
eigenvector is the first order Glauber correlation function, bridging the gap
between photon counting and the sensitivity of light detectors to
electromagnetic energy density.
