Linear response of a pre- and post-selected system to an external field

•The linear response theory is applied to a quantum system with pre-selection and post-selection.•It is found how an external field affects strong and weak values of a measured system observable•Not only an external field before measurement but also one after measurement influences the linear response due to the post-selection.

Linear response to an external field is studied for a quantum system with pre- and post-selection. Effects of an external field on strong and weak values of a system observable are found. The external field applied after the measurement of the observable influences the linear response of the system through post-selection. A time-symmetric property in the linear response is found.