  Single-photon wave packets can carry quantum information between nodes of a
quantum network. An important general operation in photon-based quantum
information systems is blind reversal of a photon's temporal wave-packet
envelope, that is, the ability to reverse an envelope without knowing the
temporal state of the photon. We present an all-optical means for doing so,
using nonlinear-optical frequency conversion driven by a short pump pulse. This
scheme allows for quantum operations such as a temporal-mode parity sorter. We
also verify that the scheme works for arbitrary states (not only single-photon
ones) of an unknown wave packet.
