  The possibility to manipulate the wavepackets of single photons or biphotons
has enriched quantum optics and quantum information science, with examples
ranging from faithful quantum-state mapping and high-efficiency quantum memory
to the purification of single photons. Here we demonstrate another fascinating
use of wavepacket manipulation on restoring quantum interference. By modulating
the photons' temporal wavepacket, we observe the revival of post-selected
entanglement that would otherwise be degraded or lost due to poor quantum
interference. Our study shows that the amount of the restored entanglement is
only limited by the forms of modulation and can achieve full recovery if the
modulation function is properly designed. Our work has potential applications
in long-distance quantum communication and linear optical quantum computation,
particularly for quantum repeaters and large cluster states.
