Enhancing quantum entanglement for continuous variables by a coherent superposition of photon subtraction and addition

We investigate how the entanglement properties of a two-mode state can be improved by performing a coherent superposition operation ta ̂ \^+ra ̂ \^^ of photon subtraction and addition, proposed by Lee and Nha [Phys. Rev. A 82, 053812 (2010)], on each mode. We show that the degree of entanglement, the Einstein-Podolsky-Rosen--type correlation, and the performance of quantum teleportation can be all enhanced for the output state when the coherent operation is applied to a two-mode squeezed state. The effects of the coherent operation are more prominent than those of the mere photon subtraction a ̂ \^ and the addition a ̂ \^^ particularly in the small-squeezing regime, whereas the optimal operation becomes the photon subtraction (case of $r=0$) in the large-squeezing regime.