Impossibility of deleting an unknown quantum state

A photon in an arbitrary polarization state cannot be cloned perfectly(1,2) . But suppose that at our disposal we have several copies of a photon in an unknown state. Is it possible to delete the information content of one or more of these photons by a physical process? Specifically, if two photons a re in the same initial polarization state, is there a mechanism that produc es one photon in the same initial state and the other in some standard pola rization state! If this could be done, then one would create a standard bla nk state onto which one could copy an unknown state approximately, by deter ministic cloning(3,4) or exactly, by probabilistic cloning(5,6). This could in principle be useful in quantum computation, where one could store new i nformation in an already computed state by deleting the old information. He re we show, however, that the linearity of quantum theory does not allow us to delete a copy of an arbitrary quantum state perfectly. Though in a clas sical computer information can be deleted (reversibly) against a copy(7), t he analogous task cannot be accomplished, even irreversibly, with quantum i nformation.