  We propose an experiment where a photon is first cloned by stimulated
parametric down-conversion, making many (imperfect) copies, and then the
cloning transformation is inverted, regenerating the original photon while
destroying the copies. Focusing on the case where the initial photon is
entangled with another photon, we study the conditions under which entanglement
can be proven in the final state. The proposed experiment would provide a clear
demonstration that quantum information is preserved in quantum cloning. It
would furthermore allow a definitive experimental proof for micro-macro
entanglement in the intermediate multiphoton state, which is still an
outstanding challenge. Finally, it might provide a quantum detection technique
for small differences in transmission (e.g., in biological samples), whose
sensitivity scales better with the number of photons used than a classical
transmission measurement.
