 After storing or erasing the cell, the controller can measure the state of the cell by passing a small voltage across the source-drain nodes; if current flows the cell must be in the "no trapped electrons" state, which is considered a logical "1".If no current is seen the cell must be in the "trapped electrons" state, which is considered as "0" state.The needed voltages are normally about 2 V for the erased state, and around 4.5 V for the programmed state.