  Frequency translation of single photons while preserving their quantum
characteristics is an important technology for flexible networking of photonic
quantum communication systems. Here we demonstrate a flexible scheme to
interface different-color photons using an optical single sideband (OSSB)
modulator. By changing the radio-frequency signal that drives the modulators,
we can easily shift and precisely tune the frequency of single photons. Using
the OSSB modulator, we successfully erased the frequency distinguishability of
non-degenerated photon pairs to obtain the Hong-Ou-Mandel interference with a
visibility exceeding 90%. We also demonstrated that the level of
distinguishability can be precisely controlled by the OSSB modulator. We expect
that the OSSB modulator will provide a simple and flexible photonic interface
for realizing advanced quantum information systems.
