Phase sensitivity enhancement for the SU(1,1) interferometer using photon level operations

We propose to employ photon level operations (PLOs) to enhance the phase sensitivity of the SU(1,1) interferometer (SUI). Two types of PLOs are considered, including photon subtraction scheme (PSS) and photon addition scheme (PAS). By performing either PSS or PAS on the two arms inside the SUI, we demonstrate that the phase sensitivity of the SUI can be enhanced. More interestingly, we find that both PSS and PAS are able to increase the robustness of the SUI against the internal loss of the interferometer, which is one of the main imperfect factors that limit the performance of the SUI.