Stable and polarity-switchable free charges are written on the surface of a LaAlO3/SrTiO3 heterostructure using conducting atomic force microscopy. These charges reversibly tune the conductance at the LaAlO3/SrTiO3 interface in a non-volatile way. The modulation in carrier density is large, approximate to 3 x 10(13) cm(-2), comparable to the maximum modulation achieved by the normal field effect.