Photon subtraction
= Photon subtraction
{title2=$|\psi_-\rangle=a|\psi\rangle/\sqrt{\langle N\rangle}$}
Ideal conditional application of an <annihilation operator>, defined only for positive initial mean <photon number>. Number-dependent weighting means that the conditional mean need not fall. Vacuum input gives the zero vector and therefore no successful conditional state.