On quantum operations of photon subtraction and photon addition

Abstract

The conventional photon subtraction and photon addition transformations, $\varrho \rightarrow t a \varrho a^{\dag}$ and $\varrho \rightarrow t a^{\dag} \varrho a$, are not valid quantum operations for any constant $t>0$ since these transformations are not trace nonincreasing. For a fixed density operator $\varrho$ there exist fair quantum operations, ${\cal N}_{-}$ and ${\cal N}_{+}$, whose conditional output states approximate the normalized outputs of former transformations with an arbitrary accuracy. However, the uniform convergence for some classes of density operators $\varrho$ has remained essentially unknown. Here we show that, in the case of photon addition operation, the uniform convergence takes place for the energy-second-moment-constrained states such that ${\rm tr}[\varrho H^2] \leq E_2 < \infty$, $H = a^{\dag}a$. In the case of photon subtraction, the uniform convergence takes place for the energy-second-moment-constrained states with nonvanishing energy, i.e., the states $\varrho$ such that ${\rm tr}[\varrho H] \geq E_1 >0$ and ${\rm tr}[\varrho H^2] \leq E_2 < \infty$. We prove that these conditions cannot be relaxed and generalize the results to the cases of multiple photon subtraction and addition.