  We show that information in quantum memory can be erased and recovered
perfectly if it is necessary. That the final states of environment are
completely determined by the initial states of the system allows that an easure
operation can be realized by a swap operation between system and an ancilla.
Therefore, the erased information can be recoverd. When there is an
irreversible process, e.g. an irreversible operation or a decoherence process,
in the erasure process, the information would be erased perpetually. We present
that quantum erasure will also give heat dissipation in environment. And a
classical limit of quantum erasure is given which coincides with Landauer's
erasure principle.
